Open the chuck fully and it shoudl have a screw inside. Unscrew it, but be carefull, these screws are usually reversed - to uncsrew, rotate it clockwise. Once done, put allen key into chuck (short side in), fasten the chuck to hold allen key well and hit the key with hammer - counterclockwise direction. You might want to verify this procedure with someone else as I have no experience with Dewalt, but this way it works for Makita, Metabo, Narex, ... then why not for Dewalt? Juraj