How do you change a thermostat on a 2002 gmc envoy 4.2?

To change the thermostat on a 2002 GMC Envoy 4.2, first, ensure the engine is cool, then disconnect the negative battery cable. Drain the coolant from the radiator and 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 everything in reverse order, making sure to refill the coolant and check for leaks. Finally, reconnect the battery and start the engine to ensure proper operation.