Use a hammer and chisel or a fine saw or cut out the damaged portion of the floor. If you can’t get it out with your hands, use a flat pry bar.
Measure the space that needs to be replaced and mark the new floorboard to fit into the space.
Cut the new piece and apply adhesive to the subfloor.
Be sure to line up the tongue and groove with the adjacent floorboard and then set the piece in place. Use a hammer and a scrap wood block to tap it into place.
If you must nail the floor down, be sure to fill all nail holes. If the color of the new board doesn’t match exactly, use a stain and let it sit over night.