Just loosen the cable (screw in the barrel adjuster) until it shifts correctly. If you go too far and need to tighten it, first shift into high to relieve tension in the cable so you don't strip the barrel adjust threads, and to make it easier.

Start with one turn, repeat, then when it gets close use half and quarter turns. Often the final adjustment is a quarter turn.

There's probably a second barrel adjuster at the derailleur.