How do you replace the thermostat on a 1999 lumina?

To replace the thermostat on a 1999 Chevrolet Lumina, first, ensure the engine is cool and disconnect the battery. Drain the coolant from the radiator, then remove the upper radiator hose connected to the thermostat housing. Unbolt the thermostat housing to access the thermostat, replace it with a new one, and reassemble by reversing the steps. Finally, refill the coolant and check for leaks.