Moved house? You can change your address in the 'My details' section on either M3 or the Three app. Just select Update address, then put in your postcode.
Choose the right address from the list provided, then hit Select address and Confirm. If you can't see your address, you can enter it manually. You can also update a British Forces Post Office address from here.
Unfortunately, you can't use a PO Box as your billing address. You can add a temporary foreign residence as a backup address, but you'll still need to be a UK resident.