If you have the 2.2 Echotec engine, it's a pain in the . . . On the back of the engine where the exhaust manifold comes out you will find the water pump on the passenger side rear of the engine. There is a cross pipe that goes under the exhaust manifold to the drivers side of the engine. There are two 10 MM bolts that hold it on the thermostat housing. Very hard to get to unless you have a stubby 10MM wrench. You will need to remove the heat shield off the exhaust manifold to reach it. Once you have the bolts out you need to twist the pipe counter clockwise to flex it past the thermostat housing and remove it from the water pump. It has a gasket on each end that you will need to replace, these are slip in compression gaskets. You can use coolant to lube it when you put it back together.
The thermostat is inside the part you unbolted the pipe from and is held in by a plastic ring. these parts are keyed so they will fit back together properly. You will need to replace the face gasket on the mating surfaces of the t-stat housing as well. Good Luck