Most decks have a bolt on one side or he other on the linkage for the deck. Should have 3 lines on the linkage for a gauge.
But before you adjust the deck, check your tire pressure and make sure they are right. To check the deck, i usually find a level surface to park the mower on. I use the floor in my garage, and then just measure from the floor to the bottom of the deck front and back, on both side.
If you are mowing a little high on one side, check the tire pressure
i spent the majority of an afternoon fighting with my mower deck thinking my kid hit something and knocked it out of level, when all it was, was a low tire