Step 1: Download form AR-11 from the USCIS website and send it to the proper address listed in the form
Step 2: Call USCIS customer service, tell them that you have sent the AR-11 form and also ask them to change the address on all existing applications (485, 140, EAD, AP, etc.)
P.S. Strongly advice you to send the AR-11 form via couriers and preserve the receipt stabs and tracking info.