My battery needs to be replaced in nest thermostat 3rd gen., ive only had it for 4.5 years, battery will stay charged for about a week and then the thermostat shuts off, had an hvac company come out thinking it was the heater, but everything tested out good, he removed the thermostat of the wall and charged the unit and the system worked, how do i replace the battery? product should last more the 4.5 years
