Heater fan works good but stuck on the vent settings can't change it to any other settings

If you have an electronic controller, that's a high-failure computer. If you have the common sense switches and levers, a cable could be disconnected, but more commonly, if you have push button controls, the mode doors are operated by engine vacuum. Look for a small hard plastic vacuum hose that's disconnected on the engine or is cracked. Normally when the vacuum supply isn't working, the system is spring-loaded to default to the defrost setting so at least the windshield will be clear.