Once you get the case back off you need to remove the movement which involves detaching the stem and removing the screws holding the movement in place. Once you have the bare case there's a press which removes and then forces the new crystal into place (it's typically a friction fit) and you need the right tool because if uniform pressure isn't applied you can distort the case. Just my 2¢ and I'm no watch repair guru, but I have seen it done more than once!