Well, assuming it's too low... you can cover up part of the lid, which might do it for a ball. You can also make one of the hides a humid hide, which means that you put something that holds humidity in inside the hide and mist that down directly. Green moss and peat moss work well for me. Increasing the size of the water dish can also help. I'm a little rusty on my ball pythons but I'd aim for about 60%. You don't want the humidity too high, and you also don't want wet substrate. So it's kind of a balancing act. You can get very nice digital temp/humidity gages for about 10$ and they're pretty handy. Symptoms of your humidity being too low include wheezing, dented eyecaps, and bad sheds. I just wanted to give you a heads up because it's a mistake I made more than once when I started getting into boas and pythons. I've got some really picky ones that need it around 80% and I had to fold and get them plastic cages. You shouldn't have to go that far, though.
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"The serpent crams itself with animal life that is often warm and vibrant, to prolong an existence in which we detect no joy and no emotion. It reveals the depth to which evolution can sink when it takes the downward path and strips animals to the irreducible minimum able to perpetuate a predatory life in its naked horror."
Alexander Skutch