Press the up and down arrows simultaneously for 3 seconds and let go. You will see F for Fahrenheit or C for Celcius. Either press the light button or wait 5 seconds and the screen will change to the differential. Adjust the differential with the up and down arrows to desired setting. It is adjustable from 1-3 in .5 degree increments. When finished leave the thermostat alone and it will return to the main screen in 10-15 seconds.