Bash with the claw of a hammer the outside corners surrounding the window to chip away the joint compound covering the corner bead. If after removing the compound you notice the installers used screws, use a screw gun to remove them and pull off the corner bead by hand. If the installers used nails, break the drywall to make enough space to pry off the corner bead.
Bash and break the drywall around the window with the claw of the hammer. Pull off the pieces until you’ve removed enough to get the claw of the hammer underneath an edge of the corner bead.
Pry up the edge of the corner bead with the claw of the hammer. Some of the installed nails will pull out, allowing you to use the claw of the hammer to remove them.
Grasp the corner bead tightly with a pair of pliers and yank it hard to loosen more nails. Continue loosening the corner bead by yanking it with pliers and then removing the protruding nails with the hammer.
Remove any remaining sections of drywall by breaking them with a hammer and then pulling them off.