Confirm that the Dewalt drill's trigger is in the lock position. Also remove the slide-on rechargeable battery.
Grab the chuck sleeve and turn it to the left , then remove the bit if there is one in place.
Slip the short end of a 1/2-inch Allen wrench into the chuck until it is flush against the back of the chuck. Tighten the chuck until it will not turn any more and the Allen wrench is firmly secure.
Use a hammer to tap the Allen wrench to the right, or clockwise, to loosen it. Chuck threads are reverse -threaded, so you have to turn it to the right to loosen it.
Twist the chuck off after you loosen it.