To replace the part you first need to attach the sticky tape. It comes in the correct outlines. Take the inner parts out to keep those areas free (Camera, Flash, Loudspeaker and a hole).
Before you take the film off practice the attaching. You will have it hard to reposition after its attached. Same goes with the lens itself, practice before the tape film is off. Once you feel safe to go, peel the film off and go for it!