On my ZFs the back crystal is part of the rear bezel; they may have updated this since but I'm not directly familiar with that.
If the construction is like the gen, there would be a metal crystal surround into which the crystal was inserted, and the metal surround with crystal in it can be removed with a pair of tweezers since it's just set into the case. If you can supply some pics we can probably figure it out, since that's not like the ZFs I've torn apart.