How do you change a thermostat on a subaru impreza RV 2005?

To change the thermostat on a 2005 Subaru Impreza, first ensure the engine is cool and disconnect the negative battery cable. Drain the coolant and remove the upper radiator hose connected to the thermostat housing. Unbolt the thermostat housing, replace the old thermostat with a new one (ensuring proper orientation), and reassemble everything by reversing the disassembly steps. Finally, refill the coolant and check for leaks after starting the engine.