The model number is TP-WEM01-A, installed with the "large backplate", as pictured below.
There are no visible screws, etc. I have the manual but it is no help.
The thermostat is simply pressed onto the mounting bracket and "snaps" into place.
Since there is no description for removal, I'd try prying gently along the four edges, between thermostat and wall.
BTW, the owner's manual omits mounting the device.