Remove the 2 #0 Philips screws securing the back of the case to the frame. The #0 screwdriver is the only one that will be needed for completion of this repair.

BEFORE GOING FURTHER - Keep in mind that the ability to drop this phone in water (without ruining its internals) WILL BE LOST, as the gasket will definitely become warped or broken during its removal

Start with your thinnest opening tool (mine happens to be a sharpened guitar pick) and insert it between the back casing and the frame. You should hear the sound of adhesive letting go.