The dimmer switch will have two screws on the back. One on one of the upper corners and one on the opposite lower corner. The switch is fastened to the plastic dash panel that surrounds it. You can get to it with a stubby Phillips head screw driver, but if you have air-conditioning you have to remove the duct first. Even then it's going to be a bit of a pinch And easier to reach with the drivers seat out of the car, but it just depends on how flexible you are. Once you get it outthe dash there should be enough wire attached for you to pull it down below the dash and work on it.

If the switch is bad, which a lot of them do, I recommend Jim at J.S. Restorations. He's a member here and he does fantastic work.