The round knob is a 3 position spring loaded switch, pull the knob all the way out and turn it to the right, the more you turn the knob, the brighter the hud display will get.
Hope this helps.
Jeff
Let me give you one 'heads up' for the HUD. If you are wearing polarized sun glasses, you won't be able to see the HUD at all no matter what you do with the brightness nob.