This repair has been made intentionally difficult. You will either need to heat up the glue to soften it and hack away at the glass, or freeze the back and chip it off. This can damage your phone, so take everything out of the housing first. You will then need to clean the glue off the frame, and then re-glue it back on (the exact glue I am not sure of). However! The glass to replace it with does not exist yet, and when it does, it will either be blank or be breaching copyright laws (as it has the apple logo on it). It cannot be done at this stage, and (I can’t believe I am suggesting this) either live with the cracks, use a skin or a case to protect it or if you really want it fixed, give it to apple.