I'm not sure what kind of computer you're on, but most laptops (if not all) have a button or a Fn+button that can change the screen brightness. On my Dell, it's on the up/down arrows. For monitors (desktops) there will usually be some option buttons that deal with brightness, contrast, screen position ext. on the front or side of the monitor.
Or you can go into settings and change the options on the video card. But for a quick change I always just use the function buttons.
Hope this helps
[This message has been edited by Renegade blob (edited 07-30-2011).]