Turn off the power to your compressor and furnace.
Take off your existing thermostat’s cover. (You may have to remove screws or other locking devices first.) Once the wallplate is exposed, locate the wire terminals and try to identify the wire designations. If you can’t see the wire terminals, they may be connected to the back of the wallplate. Look for any additional screws, tabs or locking devices to remove the wallplate. If you still can’t find the wire terminals, contact the manufacturer of your existing thermostat.