Replacing a room thermostat only takes a few minutes. It does not require any electrical skills or carpentry skills, other than the ability to use a couple of common tools. As long as the new thermostat will fit in the old opening, replacing a thermostat can be done in about the same time it takes to cook macaroni and cheese.
Before you buy a new thermostat, make sure that you are getting one which is compatible with your heating and cooling system. Except in some extreme cases, this should not be a problem. Even when modern features are added to the thermostat, it still connects to the HVAC system the same way, and uses the same wires. Of far greater concern is that the new thermostat will fit in the old location. Measure the existing thermostat, and use the numbers as a guide for the size and shape of thermostat to shop for.
Remove the Old Thermostat
For safety, turn of the circuit breaker for the room where the thermostat is installed, as well as to the HVAC unit. Unless the existing thermostat is quite old, it probably snaps into place or twists on a locking plate. Remove the face carefully, and allow it to dangle by the wiring. There should be a plastic or metal plate behind the thermostat to which it connects, and which protects the wiring from damage. Remove this plate, which is usually held in place by 2 to 4 screws.
Disconnect the Wires
Note where the wires connect to the old thermostat. Take care to note exactly which color wires connect to which thermostat controls. If the wires are labeled or connect to labeled parts, notate those labels as well. Matching wires back up to where they should go is the most difficult part of the job, and even that is only a matter of a moment if proper notes are made before hand.
Connect the New Thermostat
Connect the wires for the new thermostat exactly as they were connected in the old one. If there is a discrepancy between the old and new connections, resolve it by calling the manufacturer and explaining your problem. You will not be the first person to have this problem, and a company technician will be able to walk you through sorting it out in a very short time.
Replace the Mounting Plate
Your new thermostat came with a mounting plate or bracket. It should screw into place, covering the opening where the old mounting plate had been. Make sure that you have the wires threaded through the plate correctly, and that none of them will be crimped or pinched when the new thermostat has been set in place.
Attach the Thermostat
Snap the new thermostat into place on the base mounting plate. Turn the power back on to the room and to the HVAC unit. Set the thermostat for a low setting and operate the unit. Set the thermostat for a higher heat, and test that. If the HVAC doesn't work, or the thermostat does not control it properly, check your wiring for accidentally crossed connections, and test it again.