All work to be done by licensed professional. WE KEEP GOING UNTIL YOU HAVE THE INFORMATION YOU NEED, I come and go. This is step ONE.
Hello, welcome to Just Answer ! Only the programmable models need a back up battery.. the non programmable models do not have a battery.
If you can get the full model number of the thermostat I can be more specific...
You will also know if you have to program that thermostat, or if you merely have to set the temperature you want it to control at.
Let me know, we can go from there.