How do you change thermostat on a 1999 subaru impreza turbo?

To change the thermostat on a 1999 Subaru Impreza Turbo, first, ensure the engine is cool and then disconnect the negative battery cable. Drain the coolant by removing the radiator drain plug, then locate the thermostat housing, which is usually near the engine block. Remove the housing by unscrewing the bolts, take out the old thermostat, and replace it with a new one, ensuring the gasket is in place. Finally, reassemble the housing, refill the coolant, and reconnect the battery.