At the circuit breaker panel, shut off the power to the furnace.
Remove the thermostat cover and locate the wires from the furnace, then disconnect them.
Label each wire with the labels supplied with the new thermostat.
Remove the old thermostat from the wall. Spread the furnace control wires apart to keep them from falling behind the wall.
Remove the base from the new thermostat and position it on the wall where the previous thermostat was located. Use a level to make sure it is straight so the internal sensor will be accurate.
Mark the wall through the mounting holes on the base.
Remove the base and use your marks for reference to drill holes for mounting. Use a drill bit slightly smaller in diameter than the screws you'll be using.
Stuff the hole around the furnace control wires with a piece of paper towel. This prevents drafts, which could affect the thermostat's performance.
Feed the wires through the base of the thermostat, and screw the unit into position. If you're mounting the thermostat to drywall, use the drywall anchors supplied with the new thermostat kit.
Attach the labeled furnace control wires to the corresponding terminals on the base. Follow the new unit’s wiring instructions carefully -- color codes may not apply to your wiring.
Install the thermostat body to the base.
Program the unit according to the manufacturer’s instructions.