How to fix the humidity level in a finished basement?

Buy a dehumidifier. They aren't the cheapest appliance but they are worth every penny spent. There's auto shut off styles or remove the plug in the container and run a hose to the main drain for continuous operation or humidity control selections for auto shut off and different size containers(very large ones-but they are hard to remove when full because of the weight to dump the water out.. Try Craigslist to purchase a used good one.