Step-by-step guide :
On the back of the Room Thermostat, there is a row of white DIP switches
Put switch 1 (the left most switch), to the UP position
Rotate the dial on the front of the Room Thermostat, until 'P' is displayed on the LCD screen
Press the dial twice in succession
The LCD screen will read 'PP'
The Room Thermostat is now in a factory reset default state
Please see this article that gives you directions on including new devices using the 'advanced page'.
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'