Yes, you can request to reschedule your appointment, if you cannot make it to ASC on the specified data and time. The process is simple, you make a copy of the Appointment letter for your records, check the box as listed in the ASC Appointment Letter and mail the Original letter to  address BPU, Alexandria ASC, Suite 100, 8850 Richmond Hwy, Alexandria, VA 22309-1586. Also, if you have open wound or bandage/casts, when you appear for appointment, USCIS may reschedule the appointment if they believe that they will interfere with taking the biometrics.