Set the drill's direction indicator to reverse (to the left of the trigger) and set the speed to low.
Put on work gloves. Hold the drill chuck's collar firmly in one hand, and depress the trigger about halfway with the other to loosen the chuck. Continue reversing the chuck until the chuck collar stops turning. Remove any drill bit from within the chuck's jaws.
Insert an Allen wrench through the jaws and into the screw in the back of the chuck. Turn the Allen wrench clockwise to loosen and remove the screw.
Hold the chuck collar firmly once again in one hand and depress the trigger with the other hand to loosen the chuck from the drill's drive shaft. If the chuck spins in your hand, hold onto it with a large pair of adjustable pliers.
Spin the loosened chuck counterclockwise until it is removed from the drive shaft's threads.