To factory reset a Room Thermostat:

The Room Thermostat is now in a factory reset default state, to add it back onto the system again see this link.

 

Please note that once the device is included, it must be assigned to a room by going to the 'setup' page, selecting the relevant room, and then pressing 'Add a device'. You can then choose the new device and press OK to assign it to the room.
You just need to add the device again as if it is a brand new device to the 'Whole House'