How do I replace the batteries in the T4 Pro thermostat?
When the low battery icon appears, press gently to loosen the thermostat and carefully pull it from the wall mount. Insert fresh alkaline AA batteries and reinstall the thermostat. If batteries are installed within two minutes, the time and date will not need to be reset. (Page 6)
From the official Honeywell T4 Pro Thermostat manual, page 6
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