How do you change a drill chock on an electric drill?

First check if your tool has a screw inside the mouth of the chuck. If you find one, unscrew it clockwise (the opposite way you would usually unscrew something). If the jaws are still working you can place a L shaped hex key (around 3/8" in size) inside the mouth of the chuck and try to tighten it as best you can. Pace the drill on a firm table and with a hammer make a quick and hard hit on the part of the L that is sticking out so the chuck rotates counterclockwise. If you do this for about 5 times you should be able to loosen it enough to take the chuck off with your hand.