How do you change thermostat on a ej20 Subaru?

To change the thermostat on a Subaru EJ20 engine, first, ensure the engine is cool and disconnect the negative battery terminal. Drain the coolant from the radiator and remove the upper radiator hose from the thermostat housing. Unbolt the thermostat housing, replace the old thermostat with a new one (ensuring the gasket is in place), and reassemble the housing and hoses. Finally, refill the coolant, start the engine, and check for leaks.