Hi, Jackie. You should be able to remove the frame. There are many different manufacturers of these so the instructions might have to be altered for your particular installation.
First, remove the top rail. It should snap in place or be held with a couple of screws on each end. Remove the screws or pry around and it should come loose. Now remove the sides. They are probably held by a couple of screws into the wall. If they are rusty and you can't get them loose, you might have to take a hacksaw blade and cut them at the wall. Now remove the bottom. This should just be held by caulking. Take as big a screwdriver as you have and a block of wood and pry up with the wood under the screw driver. Then remove the remaining caulk with a single edged razor blade in a holder. These are really cheap at the home centers. You might also be able to use scouring powder and a sponge to get the any residue off. If you were able to get the screws loose, try to pull out the expansion shields (those little plastic circles that the screws went into) with pliers. Finally, fill any holes with matching grout or silicone caulk. That should do it.