To tighten the cable, place the derailleur in the 1st gear position. This position requires the least amount of tension on the cable.

Loosen the 6mm hex nut using an Allen wrench just enough to allow the cable to come free.

Pull the cable using your hands as tight as you can. Replace the cable under the screw and tighten the screw by turning it clockwise.

Repeat Step 2 to set the upper bound on the derailleur. Now that the cable is tight, the derailleur will be able to travel further away from the frame of the bike and line up with the 3rd gear.