Disconnect the power to the thermostat at the main circuit breaker.
Remove the faceplate of the old thermostat. The face plate is usually not screwed into the base plate or thermostat and will snap or twist off.
Disconnect the wires from the thermostat by turning the screws on the connectors. Do not allow the wires to fall back into the wall. You may need to pull them further out of the wall and can tape them to the wall to hold them in place.
Remove the screws holding the old thermostat and base plate to the wall. Remove the old thermostat and base plate.
Hold the new thermostat base plate in place on the wall and mark the screw holes with a pencil. Set the base plate aside.
Drill holes for the wall anchors that came with the new thermostat, and lightly tap the anchors into the holes with a hammer.
Position the new base plate on the wall, pulling the wires through the hole on the base plate. Secure the plate to the wall with the screws provided with the thermostat. Be sure to drill the screws straight into the wall anchors to avoid damaging them.
Attach the wires to the connectors, or terminals, on the new thermostat. If the thermostat is connected to both the heating and cooling system, you will have four wires. If the thermostat controls only heat, you will connect two wires. Follow the manufacturer's directions to properly connect the wires. You may need to match colored wires to the same color terminal, or you may need to follow guides to connect colors to letter-designated terminals. Wrap the end of the wire around the appropriate terminal screw and tighten the screw to hold the wire in place.
Install batteries in the new thermostat if necessary and snap the new face plate to the base plate.