Answer:
I once saw instructions on a package of a new chuck. Lock the drill in a vice, open the chuck. It should have an Allen screw inside, behind the teeth of the chuck. Find the right wrench and tap with a hammer. I can't remember which way to tap. Some items like that have reverse thread. If you have t handle allens, it will probably be easier, may be able to do it by hand. I haven't tried it, don't know how easy it is. Good luck.