Determine how much of the drywall will have to be removed, and draw lines around it using a pencil and a straightedge. This will provide a guide for cutting as well as make it easier to replace the drywall.
Score along the lines you drew with a utility knife. Hold the utility knife in your dominant hand and place your other hand on top of your dominant hand to provide more control of the knife. Do not try to cut all the way through on the first pass.
Cut along the scored lines with the utility knife several times more until the drywall is able to be removed. Use a flat pry bar to remove drywall and nails stuck to studs, and use a screwdriver to remove screws if they were used to fasten the drywall.