Press the two left buttons at the exact same time. (the 10 and 1000 button I believe). It will say "Enter Code:".
At that point, enter the number you want (1-30'ish) and hit set to go to that setting. For example, if you enter "1", and hit set, it'll do a pixel test, and light up every pixel and light on the display. Hit Check to go back to a normal mode again.