Scrape the grout and/or caulk away from the edges of the surround with a grout removal tool. Start at the bottom and work your way up. You may need to dig a little with this tool to loosen up the grout. Once you get a firm grip on the end piece, pull upwards. If the grout isn't old, it will peel off in one strip. Otherwise, keep scraping until all the grout is removed.
Insert a pry bar into the top corner between the surround and the wall. Work the pry bar back and forth gently until a few inches are hidden from view. Lever the pry bar outwards to pop the edge of the surround off the wall.
Pry the rest of the side up. Loosen the bottom first and then the middle of the surround along the edge. Repeat with the other side of the surround.
Pull the surround away from the wall. If it sticks, insert the pry bar into the area and pry outwards until it pops off. Remove the surround from the room.
Inspect the wall. Depending on the strength of the seal between the wall and the surround, you may or may not have water damage or discoloration. If the integrity of the wall has not been compromised, you can sand any rough spots out and repaint the area. If the plaster is cracked or damaged, remove the debris and skim coat the wall with a new coat of plaster.