Each thermostat terminal is identified by a letter, and each of the wires is a different identifying color. Note which wire goes to each letter, and write the letter on a small piece of tape attached to each wire. The wire colors may not attach to the same terminals on the new thermostat – always follow the letter code.
Mount the new wall plate. If the screw holes don’t line up with the existing mounting plate holes, mark the new locations and drill the holes, then install wall anchors if necessary for the new mounting plate.
Attach the wires to the terminal screws, matching the letters on the wires to the letters of the new unit.
Install batteries in the thermostat and snap the cover in place.