Open the chuck as wide as it can go and shine a light into it. There is usually a screw holding the chuck in place to stop it vibrating loose. Turn the screw CLOCKWISE to remove it as it’s a left hand thread. Then when the screw is out put a large Allen key in the chuck with the long end at a right angle to the drill and tighten it down. Lay the drill on a bench or table with the chuck over the edge and hit the Allen key with a quick blow of a hammer counter clockwise to unscrew it. Hope it helps.