First, disconnect the cables from the brakes. Pull one of the levers as if you were applying the brakes. Look in the crack at the base of the lever that opened up when you pulled on it. There will be a bolt that takes either a hex key or possibly a straight screwdriver. Loosen this screw and the clamp will come loose, allowing you to slide the lever off of the handlebars.

You will likely need to get the cable out of the way to get your tool onto the bolt. This is not very hard, but difficult to explain in writing. Jiggle and press on the cable to slide it further in the direction that pulling the brake lever moves it, and it should begin to come out inside of the lever.