How do I replace batteries in the Honeywell T4 Pro thermostat?
When the low battery icon appears, gently press to loosen the thermostat and carefully pull it from the wall mount. Insert fresh alkaline AA batteries and reinstall the thermostat. If batteries are inserted within two minutes, time and date settings will be preserved. Replace batteries at least once yearly or before leaving home for more than a month.
From the official Honeywell T4 Pro Thermostat manual, page 6
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