Shut off the power to the existing thermostat by flipping the switch at the circuit breaker.
Snap the cover off from the mounting plate of the existing thermostat.
Wrap a small piece of masking tape around each of the wires in the thermostat. Write the appropriate labels on each wire. There are several wires that connect to the thermostat so they can get mixed up during installation if they are not labeled.
Unwrap the wires from the screw terminals and loosen the terminals with a screwdriver. Unscrew the mounting plate from the wall.
Screw the mounting plate for the new digital thermostat to the wall with a screwdriver. Before screwing the mounting plate in place set a level on top of the plate and adjust the plate until it is level.
Match the appropriate wires with the appropriate screw terminals in the digital thermostat. Wrap the wires around the terminals. Tighten the screw terminals with a screwdriver.
Attach the base of the thermostat to the wall. The base should simply snap into place on the mounting plate.
Turn the power back on. Set the thermostat according to the manufacturer's directions.