Six-wire thermostats are commonly seen with heating systems that have two stages of heat and one stage of air conditioning. Two stages of heat means that when the thermostat first calls for heat, the furnace will turn on using the low setting first. If the temperature is not satisfied after a certain number of minutes, the furnace will ramp up to a higher output of heat and will run at this higher setting until the temperature is satisfied. With a few tools, it is fairly easy to replace a six-wire thermostat.
Turn off power to the furnace and air conditioner by switching the correct breakers off in the circuit breaker panel.
Remove the old thermostat from the wall by gently pulling it off the wall plate. Disconnect each thermostat wire and label it with what terminal it connected to.
Remove the old wall plate from the wall and mount the new thermostat wall plate using drywall anchors and screws.
Connect each thermostat wire to the new thermostat. Make sure they are connected to the same lettered terminal that they were connected to on the old thermostat.
Turn the power back on to the furnace and air conditioner. Set the thermostat to your desired setting.