Hi, how you change that is by moving the joystick until you come to contrast in the Tray. If you scroll through all the menus & don't come to Contrast that means it's not selected to be displayed, in which case you have to push the Home button (on RHS under the stop / start rocker) scroll through there until you come to Trays, open that & you'll see a tick beside what's displayed in the Tray when you scroll with the joystick.
If required, locate Contrast & select it by pushing in on the joystick.
Then push the home button again & it'll take you back to where you were.
Now scroll through until you come to Contrast, moving the joystick up or down changes the setting (you can see what's being selected on the very right as you move it) Select either Auto or Bright & you screen will be back how you'd like it.