replacing a shower base
Aluminum shower stalls like that often have a pair of channels that mount to the wall, then the panels slip into them. A small screw or other retainer holds them in place. Look for some way to release the panels. You may need to cut a bead of caulk. 

The base is probably nailed or screwed through the top flange. You'll see it when you peel the wall panels off the drywall behind it. You'll also need to cut caulk against the floor and elsewhere for the base.