The I-751 form and accompanying documents are mailed to the USCIS Service Center that serves your geographical area. (Use certified mail or priority mail with delivery confirmation, and write "Attention: I-751 petition to remove conditions of status" on the envelope under the address.) You must mail the package to the address as directed (for the type of form) by the USCIS Service Center you are filing at. You will receive a notice of receipt for the form stating that "Your alien card is extended one year - employment and travel authorized". Keep the receipt with your green card. If by some chance you are not approved within a year you will want to make an InfoPass appointment to go to your Local Office to request an I-551 stamp in your passport. This does happen as it sometimes it takes over a year to be approved. After your NOA extension expires, this is your only evidence of legal status which you are required to have, by law. This happens occasionally, that a case takes over a year, but it's rare. Local Offices will not give an I-551 stamp is you have another type of evidence of status (ie, expired Green Card + extension letter).