There should be letters on the old unit next to each wire. If not, use a drill for new holes and use dry wall mounts if needed. How do I install a digital thermometer if one has four wires and the other has three? Warnings Follow digital thermostat installation instructions closely to avoid injury or problems. Attach the cover of the new digital thermostat to the sub-base on the wall. AND it has been recorded by a professional in the industry. Most of them simply pop off easily with a little force.