Cut a piece of corner trim 2 inches larger than the damaged area to create a patch. Remove the nailing strips from the patch with tin snips.
Sand the back of the patch and the area around the damaged section with sandpaper.
Squeeze polyurethane construction adhesive on to the back of the patch. Set the patch over the damaged corner. Wipe up any adhesive that oozes from the sides of the patch with an old rag.
Tape the patch in place with painter's tape. Let the adhesive dry to the manufacturer's recommendations. Remove the tape once the adhesive is dry.