From the owner's manual:
- Open front thermostat door and open battery compartment door.
- Install two new "AA" alkaline batteries. Locate the positive (+) ends of the batteries and match them with the positive (+) terminals located in the battery compartment. Close battery compartment.
Going by the photo of the thermostat, it looks like there's a finger recess or latch on the right side of the thermostat that you pull on to swing the front open. If there is unexpected behavior from the thermostat after replacing the battery, see the reset procedure in the installation section of the manual.
Link to manual: https://thermostat.guide/proselect/proselect-pstsl21p52-2h-1c-programmable-thermostat-installation-guide/#battery-replacement
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