Here are two instructions that I found
1) Hold menu and '+' buttons (3rd and 5th buttons from left) while powering on the unit.
2) Press menu button (5th button from left) and select "Factory Reset"
3) Select "Factory" at the bottom of the menu.
or
To enter the service menu, turn the monitor off, hold down the menu button and the + button at the same time, and while you're holding them down, turn the monitor back on. The monitor will take a second longer to come on. Once on, wait a second and then press the - button and the service menu will pop up.