To install a programmable thermostat, shut off power to the old thermostat, detach it and inspect its wiring. Label each wire with the letter of the terminal where it's attached before disconnecting them. Connect the wires to the corresponding terminals on the programmable thermostat before mounting it on the wall.
Before beginning the installation, turn off the power at the circuit breaker for the room where the programmable thermostat is located. Detach the old thermometer and its wall plate from the wall. Older model thermostats with glass tubes contain mercury, which makes it advisable to use caution when handling such models. Inspect the wiring of the old thermostat, and use masking tape to label each wire with the letter code of the terminal where it's attached. Disconnect the wires from the old thermostat, and then tape them to the wall to prevent them from falling back into the wall.
Install the wall plate for the programmable thermostat by marking and drilling screw holes if necessary. Attach the wires to the programmable thermostat by matching the letter codes on the wires with those on the new unit. Place batteries in the unit, and then mount it onto the wall plate before snapping its cover in place. Turn the power back on, and program the thermostat according to its manual's instructions.