U call them at customer service and tell them what U wish to do. They will send U what they call recovery kits and U put them into the boxes and send them back and I have never been charged anything to do this. I take mine to the post office.
Make sure U are not tied into a contract and end up owing them a lot of money. If the units are really old, they may not even want them returned. I had two HD DVR units, the HR21 and on one, they asked me to send back the access card and they sent an envelope for that and the other, they didn't ask for anything back.