The thermostat is located at the backside of the lower intake. If you look under the throttle body you will see it.
The air intake hose it about it. Its a tight fit, so you won't have alot of room for hands. Here are step by step directions.
Disconnect the negative battery cable.
Partially drain the engine cooling system, into a suitable container, to a level below the thermostat.
If replacing the water outlet, detach the radiator hose from the outlet.
Unfasten the water outlet attaching bolts and remove the water outlet.
Remove the thermostat. Remove and discard the gasket.
Thoroughly clean the manifold water inlet and water outlet mating surfaces.
Position the thermostat into the inlet manifold.
Apply a 0.125 in. (3mm) bead of RTV sealer to the thermostat housing.
Install the water outlet to the inlet manifold, then tighten the attaching bolts to 18 ft. lbs. (23 Nm).
Attach the radiator hose to the water outlet housing.
Refill the engine with the specified type and amount of engine coolant. Connect the negative battery cable, start the engine and check for coolant leaks.