The key to replacing an old mechanical Honeywell room thermostat is making sure you bought the right replacement. You can verify it by counting the number of wires coming out of the wall and attached to terminals in the old thermostat. Ignore any wires connected to terminals "C" or "C1" and count. If the total number is four or less, the job can be completed easily. If there are five or more, not including "C" and "C1," it's an unusual installation and you need to get advice from an electrician or talk to your hardware store.
Label the wires with the included tape. Use the marked tape that corresponds to the letters on the old Honeywell thermostat. Ignore colours. Use a screwdriver to disconnect the wires one by one, and don't let the wires slip back into the wall. Remove the old wall plate. Pull the wires through the new base plate.
Mark the mounting position. Level the base plate with a level, and mark the position of the screw holes.
Mount the base. Drill holes at the pencil marks. Hammer anchors into the wall, and insert the screws.
Connect the wires with a screwdriver. Loosen, connect and tighten the terminals. Match each labelled wire with the same letter on the new terminal.
Set the adjustment lever depending on the type of fuel and furnace you have. Refer to the instruction sheet that came in the kit.
Mount the thermostat on the base, and tighten the screws. Align the face plate, and snap it into place.
The kit comes with an extra large additional base plate that you can use to cover any unsightly marks left by the old Honeywell thermostat.