Loosen the Jambs
Lay a drop cloth in the tub and, using a screwdriver and a claw hammer, begin to pry loose the screws that fasten the side jamb to the wall (Image 1). With the screws removed, use a utility knife to make a cut on the outside of the jamb where the caulk adjoins the jamb to the wall; make the same cut for the inside of the jamb facing the shower.
Place the edge of a putty knife in the groove of the cuts and tap the knife end with a hammer to loosen the caulk (Image 2). When the bond has been broken, pull the jamb out from the wall.
Repeat all steps for the remaining jamb.