To replace a Nest thermostat, turn off the power to the equipment, remove the cover, and label the wires before disconnecting them and removing the base. A Nest thermostat can be replaced by another Nest model or any other programmable thermostat.
To avoid blowing a fuse, turn off the power to any equipment that is connected to the thermostat. The power can be turned off at the breaker box or by a power switch, if available. Remove the display cover, and take a picture or label the wires. New Nest thermostats include tiny labels that can be attached to the wires to identify them.
Disconnect the wires from the Nest thermostat, and then unscrew the base that is mounted to the wall. If the Nest thermostat is installed on an electrical box, a steel plate also needs to be unscrewed from the electrical box.
To install a new thermostat, pull the electrical wires through the hole in the base of the thermostat, and mount it on the wall in the same location as the old one. Connect the wires to the corresponding spots on the thermostat. Attach the display cover, and turn the power back on to program the new thermostat.