Use the Allen wrench to loosen the Allen bolt that keeps the derailleur cable in tension and attached to the derailleur body.

The Allen bolt is located near the front sprockets (gears) on the opposite side (the left side of the frame).

Over-loosening can unthread the bolt from the small nut on the other side, which is hard to get rethreaded. Try to avoid backing off much farther than the diameter of the cable.