Registration occurs during the initial pairing with the Honeywell Home app. If you receive an error message stating that the thermostat is already registered, you can contact the previous owner of the thermostat and ask that they delete the thermostat from their account, or contact Resideo Customer Care to unregister the device. (Please have the model number and MAC ID [inscribed on the back of the thermostat faceplate] to expedite the process.
*If you receive a different error during the connection / registration process, please see the When setting up my T5 / T5+, why can't I connect to my Wi-Fi? Support article for more information.