Trace the upper radiator hose to the top of the engine. Where it connects is the thermostat housing. Remove the two bolts holding that housing to the intake manifold. The thermostat sits under it, with the spring side down. Pull it out and replace with a new one, along with a new gasket. It is easier if you drain some anti-freeze beforehand, keeps it from gushing out. If it is easier, you can pull the upper radiator hose off of the thermostat housing.