Unplug your drill's power cord, or remove the attached battery pack if your drill is cordless. It is never safe to work on your drill while it is still connected to a power source.
Lock the chuck in place, and locate the Phillips-head screw securing the chuck to the chuck to the main spindle. Turn the Phillips-head screw in the clockwise direction until the screw is removed. Jacobs chucks use left-handed Phillips-head screws, which is why loosening them requires that you turn the screws in the clockwise direction instead of counterclockwise direction.
Place the drill on a work bench with the check extending past the edge of the table. Place a hex wrench into the chuck, and strike the chuck with a hammer or mallet until the chuck rotates in the counter clockwise direction. This knocks the chuck loose from the spindle. Turn the chuck in the counterclockwise direction with your hands until it completely removed from the drill.