For additional details, see here: https://forums.xfinity.com/t5/My-Account/ANSWERED-How-to-Return-Your-Xfinity-Equipment/ta-p/3003387
The information provide by @i-am-nerdburg, is correct. If you have any further questions do not hesitate to reach back out to us.
Just a heads up all, you cannot return a remote to a UPS store, I tried.
They told me they could not accept the remote, as there was nothing to scan on the remote.
OMW to a xfinity store.
GL out there,
Unfortunately at time stores will be busy, UPS stores are another option to return equipment.