Clean the lens with a microfiber cloth to remove any impurities and examine the scratches. Spray the lens using a DVD/CD cleaner and buff with a soft cotton rag to fill in the scratches.
Pop the lens back into the frames by gently pushing on both sides with your fingers until it is secured. This might not work if the frames are missing a small screw.
Take a small paper clip and unfold it with your fingers. Cut a small piece of the paper clip with wire cutters so that it fits into where the small screws was sitting. The length will be determined by the thickness of the frames. Place a small amount of super glue on the opening after inserting the paper clip part and let it dry for a few hours.