Just below the glass on the aluminum frame is a rubber gasket that runs the entire length of the windshield. Start in a corner and remove the rubber, behind it will be some Phillips head screws at an angle into the deck of the boat tighten these screws and reinstall the rubber gasket.
You may also want to remove the screws completely, one at a time and reinstall them with silicon on them. This will help to keep them tight, especially if you have a system that puts out some bass.