You can change the battery by following the steps below:
Open up the sensor by placing a small flat screwdriver in the slot located at the top of the sensor behind the facia and twisting the screwdriver.
You will be able to see the CR2023 battery inside the battery holder.
Remove the battery from the housing gently to prevent any damage to the battery holder
Replace the battery with a new one. Just make sure the battery is installed properly in the battery holder.
Once the battery is replaced, try to press the ON/OFF button and the sensor will start working again.
If the ON/OFF light starts flashing instead of going red or blue, This means that the sensor needs to be paired again with the wall control panel.
in order to do that, just press and hold the pair button for a few seconds and then release it. it should repair itself to the system.
However, if the issue persists, please follow the instructions HERE to pair the wireless temperature sensor to the wall controller and that should resolve the issue.