You may revoke an existing power of attorney by filing a new Form 2848, and not checking the box on Line 6. If you do not choose to add a new power of attorney, you may simply file a new form, write "REVOKE" across the top, and sign and date the form. From there, fax or mail a copy of the existing power of attorney to the appropriate IRS office. See the form instructions for more details. If your designated representative wishes to withdraw, he or she would file a form, writing "WITHDRAW" across the top, and follow similar instructions.