How do you change the thermostat on a 1990 Chevy 2500?

If you follow your upper radiator hose from the radiator to the engine you will see it is connected to a outlet that is bolted to the intake manifold. This is your thermostat housing/coolant outlet. Drain the coolant from your radiator, at least half way. The remove the upper radiator hose. Remove the thermostat housing. Now you can remove the thermostat. Make sure you clean all the old gasket off of the the housing and manifold. Then install the new thermostat, a new gasket )I also recommend using some rtv or gasket sealant to prep the gasket) and the thermostat housing. Then hook your hose back up. remove the radiator cap and refill your radiator. Make sure to use a 50/50 mixture antifreeze.