Press and hold both SELECT and SET buttons for 3 seconds and let go. You will see F for Fahrenheit or C for Celcius. Press and release the SET button and you will see 12 hour clock. Press and release the SET button and you will see the differential. Press and release the SET button and you will see the second stage differential. Press and release the SET button and you will see the second stage delay. Press and release the SET button and you will see the filter monitor. It is adjustable with the arrows from OFF-9900 hours in 100 hour increments. When you are finished either press the SET button or leave the thermostat alone and it will return to the main screen.