To replace an electric-heat thermostat, disconnect the electricity, remove the thermostat and its housing, and disconnect its wires. Connect the new thermostat, insert the batteries, attach the housing, and reconnect the power. The required supplies include a screwdriver, electrical and masking tape, a marker, drywall anchors and a drill.
Disconnect the thermostat's electrical supply, unscrew its housing, and pull it away from the wall. Ease the thermostat out from the wall to expose its wires. Wrap a small piece of masking tape around each wire, and mark it with the letter on the corresponding terminal screw. Loosen the screws, unwind the wires, and set the old thermostat aside.
Unscrew and remove the existing thermostat mounting plate, and hold the new one up to the wall. If the screw holes do not align with those from the previous plate, mark their location on the wall, and drill holes at those points. Insert drywall anchors into the new holes, and screw in the mounting plate.
Inspect the wires coming from the wall, and trim any that are frayed. Loosen the screw terminals on the new thermostat, and wrap each wire around the corresponding screw. Refer to the tape labels as necessary. If there are more wires than screws, cover the ends of the unused wires with electrical tape, and push them into the wall hole. Tighten the screw terminals, insert the thermostat batteries, and push the thermostat into the wall. Snap it into the mounting plate, and restore the electricity.