<PAGE>

                                                                 Exhibit 10.26.2

                               February 11, 2000


PaeTec Corp.
290 Woodcliff Drive
Fairport, New York   14450

     Re:  Lease Amendment/PaeTec Building, 600 WillowBrook Office Park

Gentlemen:

     The purpose of this letter is to amend the Lease Agreement dated July 7,
1999, (the "Lease") between WillowBrook II L.L.C. ("Landlord") and PaeTec Corp.
("Tenant") for space at 600 WillowBrook Office Park, Fairport, New York.  I have
attached a copy of the facing page of the Lease to this letter.

     The Lease is hereby amended as follows:

     1.  Section 1.01 of the Lease is hereby deemed eliminated and is hereby
         declared null and void and in its place and stead shall be substituted
         the following:

             Section 1.01  Landlord will construct a development on certain land
             ------------
             which will contain parking areas, common areas and a building,
             known as Building 600 (the "Building") WillowBrook Office Park (the
             "Park") containing approximately 98,717 rentable square feet.
             Landlord shall construct the Building and associated roadways,
             access road, parking lot (which shall contain 362 parking spaces),
             common areas, and landscaped areas in a good and workmanlike manner
             in accordance with the drawings of David Hanlon, Architect, and
             Charles J. Costich, P.E.; which drawings are attached hereto,
             labeled in the aggregate Exhibit A, and shall become a part hereof.
             Tenant, subject to any required governmental or quasi-governmental
             approvals, not later than the first day of the 15th year of the
             demised term, by written notice to Landlord, may require Landlord,
             at Landlord's sole cost and expense, subject to the approvals of
             any governmental or quasi-governmental board, bureau, or agency
             having jurisdiction thereof, to build an additional 38 contiguous
             parking spaces where said spaces are shown on Exhibit A and
             identified thereon as "Possible Future Parking".

     2.  The reference on the bottom of Page 1 of the Lease "**See Rider 1,
         Paragraph 63" shall be deemed eliminated.
<PAGE>

     3.  The last sentence of Section 1.02 of the Lease shall be deemed
         eliminated and of no force or effect.

     4.  Section 1.03A of the Lease is hereby deemed eliminated and is hereby
         declared null and void and in its place and stead shall be substituted
         the following:

             Section l.03
             ------------

             A.  The Demised Premises and the Additional Demised Premises shall,
                 together, include the entire Building and shall be the area
                 bounded by the line established by the exterior face of the
                 exterior wall of the Building (but not the surface thereof),
                 the concrete floor surface, and the lower surface of the next
                 higher floor (or roof); all as depicted on Exhibit A attached
                 hereto and made a part hereof.

     5.  Section 1.04 of the Lease is hereby deemed changed so that the
         reference therein to October 1, 1999, as the date for the commencement
         of construction shall be deemed changed to March 15, 2000. In the event
         of the occurance of weather conditions that render the said
         construction to commence on or before the said date of March 15, 2000,
         unreasonable, then the said date of March 15, 2000, shall be deemed
         extended until ten (10) days after said weather conditions change
         sufficiently to render the commencement of construction reasonable.

     6.  Section 2.01 of the Lease is hereby deemed eliminated and is hereby
         declared null and void and in its place and stead shall be substituted
         the following:

             Section 2.0l  Tenant covenants to pay an annual rent as follows:
             ------------

             A.  Subject to the provisions of subparagraph B below, for the
                 first lease year of the demised term, the sum of $1,296,156.96
                 ($108,013.08 per month);

             B.  For the second lease year of the demised term, the sum of
                 $1,506,973.11 payable in monthly installments as follows:

                 Month of 2nd Lease Year    Monthly Installment
                 -----------------------    -------------------

                 1st Month                  $110,715.85

                 2nd Month                  $113,418.62

                 3rd Month                  $116,121.39

<PAGE>

                 4th Month                  $118,824.16

                 5th Month                  $121,526.93

                 6th Month                  $124,229.70

                 7th Month                  $126,932.47

                 8th Month                  $129,635.24

                 9th Month                  $132,338.01

                 10th Month                 $135,040.78

                 11th Month                 $137,743.55

                 12th Month                 $140,446.41

             C.  Notwithstanding the foregoing, commencing the latter of (1) the
                 Rent Commencement date or (2) January 1, 2002, the then
                 existing annual rental shall be increased by the sum of
                                                 ---------
                 $136,219.92 ($11,351.66 per month);

             D.  For the 3rd lease year through the 10th lease year of the
                 demised term, the sum of $1,821,576.96 payable in monthly
                 installments of $151,498.08.

             E.  For the remainder of the demised term, the annual rental of
                 $2,003,025.12 ($166,918.76 per month). See Rider l,
                 Paragraph 57.

             Said rent shall be payable in advance on the first day of each and
             every calendar month during the term hereof. Rent for any period of
             less than one (l) month shall equal l/30th of the monthly rental
             for each day of such period.

         7.  Section 7.01 of the Lease is hereby deemed eliminated and is hereby
             declared null and void and in its place and stead shall be
             substituted the following:

                 Section 7.01  The Demised Premises shall be furnished with the
                 ------------
                 electrical and heating, ventilating, and air conditioning
                 services referred to in Exhibit C, Paragraphs A6 and A7 and
                 subject to any laws, regulations, or statutes applicable
                 thereto, said services shall be supplied to the Demised
                 Premises twenty-four hours a day, seven days a week, fifty-two
                 weeks a year, and domestic water for a drinking fountain on
                 each of the floors of the Building, appropriate men's and
                 ladies' room on each of the floors of the Building and separate
                 men's' and ladies'
<PAGE>

                 shower/changing area attached to and a part of each respective
                 men's' room and ladies' room on the first floor. Landlord shall
                 not be liable for failure to furnish any of the foregoing when
                 such failure is caused by the accidents or conditions beyond
                 the control of Landlord, or by repairs, labor disputes or labor
                 disturbances, of any character, whether resulting from or
                 caused by acts of Landlord or otherwise; nor shall Landlord be
                 liable under any circumstances for loss of or injury to
                 property, however occurring, through or in connection with or
                 incidental to the furnishing of any of the foregoing, nor shall
                 any such failure relieve Tenant from the duty to pay the full
                 amount of rent herein reserved, or constitute or be construed
                 as a constructive or other eviction of Tenant.

             8.  Paragraph 63 of Rider 1 to the Lease is hereby deemed
                 eliminated and is hereby declared null and void.

             9.  A.  Effective January 1, 2002, the Demised Premises shall be
                 deemed to include the Additional Demised Premises, as shown on
                 Exhibit A, consisting of 7,000 rentable square feet.

                 B.  Landlord shall, at its sole cost and expense, complete the
                     Additional Demised Premises as follows ("Additional
                     Landlord's Work"):

                     (1)  Interior finish of walls shall be gypsum wallboard
                          taped and finished with two (2) coats flat latex paint
                          with single color throughout ("non-decorator" color to
                          be selected by Tenant);

                     (2)  All Floors shall be carpeted with 26 oz. closed loop
                          carpet over felt pad. Color to be chosen by Tenant
                          from Landlord available colors. Matching 4" vinyl
                          base;

                     (3)  Ceiling shall be premium 2'x2' tegular tile, lay-in
                          panels in exposed metal tee grid. Ceiling heights to
                          be 9'0".

                     (4)  Interior drywall partitioning as requested not later
                          than October 1, 2001, by written notice of Tenant to
                          Landlord.

                              Partitions shall be 2"x 3 5/8" studs 24 o.c. with
                              l/2" gypsum wallboard, both sides, taped and
                              finished. Partitions shall be complete with flush
                              wood, solid core cherry veneer full-height doors
                              (one per room), hollow metal frames (painted to
                              match adjoining walls), trim and Yale or equal
                              polished brass lever-style passage sets. Lock-set
                              at entrances.

                              Full-height l'6" glass sidelites on "interior"
                              offices.
<PAGE>

                     (5)  Heating and air conditioning - Subject to Article 7 in
                          the main body of this Lease Agreement, Landlord shall
                          provide the following air conditioning and heating of
                          the Tenant space:

                          (a)  The Building will be heated and cooled with
                               ceiling-concealed water-source heat pump terminal
                               units.

                          (b)  Ventilation air will be introduced through a heat
                               exchanger and ducted to the return air opening of
                               each heat pump unit. Ceiling plenum exhaust will
                               be ducted to the other side of the Heat Exchanger
                               which will preheat or pre-cool the ventilation
                               air depending on the season.

                          (c)  System Design Conditions:

                               (i)   Air Conditioning Capacity:  Approximately
                                     400 sq. ft. per ton.
                               (ii)  Heating Capacity: 72 degrees Fahrenheit
                                     with outside conditions at l degree
                                     Fahrenheit, 15 MPH wind velocity.
                               (iii) Ventilation:  15 CFM/Person;
                               (iv)  Air Diffusion: Constant Volume, average air
                                     velocity of between 25 and 50 FPM, measured
                                     within the occupied space.
                               (v)   Thermostatically controlled zones to
                                     provide reasonable comfort conditions, not
                                     less than 800 sq. ft. per zone.
                               (vi)  Hours of Operation:  24 hours per day,
                                     7 days per week, 52 weeks per year.

                     (6)  Electrical:

                          (a)  Subject to any applicable law, statute, ordinance
                               or regulation, Landlord shall provide recessed
                               fluorescent light providing an average of 60 foot
                               candles. Light fixtures to be 2'x4', 3 tube,
                               "deep cell" parabolic fixtures.

                          (b)  Outlets - Up to 70 wall-mounted 110 volt 20 amp
                               duplex convenience outlets (non-dedicated). There
                               shall be 12 outlets per circuit.

                     (7)  l" Levelor (or equal) tilt-up horizontal metal blinds
                          on all exterior windows. Alabaster in color.

                     (8)  All foregoing work shall be in accordance with local
                          building codes and regulations.
<PAGE>

                 C.  Provided Tenant, not later than October 1, 2001, shall have
                     furnished Landlord with full and complete drawings and
                     specifications which are reasonable and adequate in their
                     substance, content and form to enable Landlord to complete
                     the "Additional Landlord's Work", Landlord agrees to
                     "substantially complete" the Additional Landlord's Work on
                     or before December 15, 2000 (the "Additional Demised
                     Premises Completion Date"). Unless caused by the action or
                     inaction of Tenant, Tenant's employees, agents, contractors
                     or representatives, in which case the Additional Demised
                     Premises Completion Date shall be extended one (1) day for
                     each day of delay so caused, the rental reserved herein
                     shall abate at the rate of two days rent for each day of
                     delay past the Additional Demised Premises Completion Date.


     Except as modified above, all other terms, provisions, and conditions of
the Lease shall remain unchanged and are hereby ratified and confirmed.

     If the above reflects your understanding, please sign the enclosed copy of
this letter and return it to us as soon as possible.

                                            Cordially yours,

                                            WILLOWBROOK II, L.L.C.


                                            By /s/ Charles N. Mills
                                               -------------------------------
                                                 Charles N. Mills

The above is hereby accepted and approved.

PAETEC CORP.


By  /s/ Arunas A. Chesonis
   -------------------------------
   Arunas A. Chesonis        CEO
   (Name)                  (Title)              Dated: 2/17/00
                                                      --------------------
<PAGE>

STATE OF NEW YORK    )
                     )    SS:
COUNTY OF MONROE     )

          On the 18th day of February, 2000, before me, the undersigned,
personally appeared CHARLES N. MILLS, personally known to me or proved to me on
the basis of satisfactory evidence to be the individual whose name is subscribed
to the within instrument and acknowledged to me that he executed the same in his
capacity and that by his signature on the instrument, the individual or the
person upon behalf of which the individual acted, executed the instrument.


                                            /s/ Debra Ann Phegley
                                            --------------------------
                                                   Notary Public

                                            DEBRA ANN PHEGLEY
                                            Notary Public, State of New York
                                                Monroe County
                                            Commission Expires Jan. 31, 2002


STATE OF NEW YORK     )
                      )  SS:
COUNTY OF MONROE      )

          On the 17th day of February, 2000, before me, the undersgined,
personally appeared Arunas A. Chesonis personally known to me or
proved to me on the basis of satisfactory evidence to be the individual(s) whose
name(s) is (are) subscribed to the within instrument and acknowleded to me that
he/she/they executed the same in his/her/their capacity(ies), and that by
his/her/their signature(s) on the instrument, the individual(s), or the person
upon behalf of which the indivudual(s) acted, executed the instrument.



                                            /s/ Karen F. Ferrini
                                            --------------------------
                                                   Notary Public

                                            Karen F. Ferrini
                                            Notary Public, State of New York
                                                No. 4765553
                                            Qualified in Monroe County
                                            Commission Expires Nov. 30, 2000