When your shower stall gets worn or badly cracked, you will want to replace it. Replacing a shower stall is relatively easy chore. The hardest part will be getting it through doorways. If you can get it in, then you should have no problem installing it. Don't despair if your doorways are not wide enough, though. Shower kits do come in pieces, making it easy to build your shower from scratch. Putting in a new shower stall is not difficult, but you will need someone to help lift the stall onto the drain. After that, you are well on the way to replacing the shower stall.
Turn the water valve off in the access panel for the shower plumbing. Remove the shower fixtures and drain assembly. You will need to take off the shower head and disconnect any flex hose from the water knob(s). Then take off the knobs by unscrewing the set screw on the bottom and firmly pulling them off. Unscrew pipe stems and put them aside. The drain assembly will normally have a screw directly in the center of the screen and you may need to use the utility knife to cut the plumbers putty around the outside. Then use a flat head screwdriver to pry the mechanism up. Some mechanisms also screw out.
Use a utility knife to cut through the silicone caulk all around the outside of the unit. Make sure you get all the way through or it will get hung up when you are pulling it out and could possibly tear the drywall.
Pull the unit from the top and tilt it toward you before pulling it out. This will allow you to clear the drain pipe. Take the unit out and discard.
Scrape the remaining silicone off the wall with a putty knife and sand the area to smooth it out. Wipe all the sanding dust off the walls so you don't have a problem with the new silicone sealing. Clean up all the water connections and drain area with a detergent and warm water.
Lift the unit in place and set it down so the drain opening and drain match up. Draw a line at the top and outside edge of the side where the shower head and knob will be. Take the unit back out. Measure from those lines to the shower head pipe and the water knob(s) pipe. Transfer those measurements to the shower unit and cut the holes for the pipes using a hole saw.
Place the unit back in the opening, matching up the drain again. Use Teflon tape on the threads of the stem pipes and screw them back in. Put plumbers putty around the drain assembly and reinstall it. Be generous with the putty and wipe off the excess when you're finished. It's easier to wipe some off than have to redo it because it leaks. Screw the screen back onto the drain assembly and install your shower head and knob.
Put a bead of silicone caulk around the outside of the knob trim and shower head trim. Then use your finger and smooth it down. Run a bead of silicone caulk around the entire outside of the unit where it meets the wall and the floor. Wet your finger and smooth it down all the way around.