Normally the only way to repair broken glass on a tank is to use a knife and cut the silicone seal between the pieces being left and the piece being removed. Once you remove this all of the old silicone now exposed needs to be removed from the other remaining pieces. Once you have done this you need to clean the pieces to remove any finger oils and then you need to use fresh aquarium safe silicone to seal in a new piece. Typically you will have issues sealing the new silicone where it meets the old silicone. Many people concerned with leaks will "simply" remove all of the old silicone from the entire tank and essentially rebuild the tank with the new piece of glass.