How do you replace the thermostat on 1991 V6 Toyota 4Runner?

To replace the thermostat on a 1991 V6 Toyota 4Runner, first, ensure the engine is cool, then disconnect the negative battery terminal. Drain the coolant from the radiator and remove the upper radiator hose to access the thermostat housing. Unbolt the thermostat housing, remove the old thermostat, clean the mating surfaces, and install the new thermostat with a new gasket. Finally, reassemble everything, refill the coolant, and check for leaks.