How do I change the thermostat on my 99 chevy lumina?

Under the hood, The radiator is just behind grill. There should be a large rubber hose running from the top of the radiator towards the back. The first thing it runs into is the housing for the thermostat. There are 3 or 4 bolts that need removed and the thermostat is just below the cover. Remove it and replace. The seal should also be replaced.

Some antifreeze might spill out when you take the housing cover off. Check the level of antifreeze when you are done.

On most Chevy's the thermostat is located where the top radiator attaches to the engine. If you look there you will see two bolts holding down the thermostat housing.Re move the hose from the housing first before you do anything else and tie it back out of the way. Using a ratchet rench with the proper size socket, probably 9-16's or 5-8's, loosen the bolts and take out. You may have to use a large flat blade screwdriver to break the housing away from the engine intake. Make sure you have the gasket that has to be replaced between the thermostat housing and the intake. Use a gasket sealer to assure proper seal. Make sure when replacing the thermostat that you put the spring side of the thermostat into the housing first or the temp of the hot water will not open it when your motor needs cooled off. Or just sit the thermostat into the hole on the intake with the spring side up. Then put a small bead of sealer around the hole and then the gasket, then a small bead of sealer on the gasket. And then replace the thermostat housing over the thermostat and bolt down. Make sure your gasket is lined up for the second bolt you will be putting back in before you tighten down the first., and your done.