Look for the upper radiator hose. One end is connected to the radiator, the other end is connected to the thermostat housing. Remove the hose from the thermostat housing, then remove the two (1/2 inch head) bolts that secure the thermostat housing to the intake manifold. Remove the thermostat housing, then pull the thermostat and seal from the intake. Installation is the reverse of removal.
Note that this should be done with a cold engine so that the exhaust crossover pipe does not burn you.
A new seal should always be used when replacing the thermostat.
Many aftermarket thermostat seals may may fit poorly and cause leaks.
RTV can be used to hold the new thermostat in place while fitting the thermostat housing over it. Don't use so much of it as to restrict coolant flow or jam up the new thermostat. Use only a small dab.
The thermostat housing mounting bolts can be difficult to access. A ratcheting box wrench will be very very helpful. If a ratcheting box wrench is unavailable, a stuppy box wrench can be used, or you can remove the exhaust crossover pipe to improve access.
The easiest way to replace the thermostat is with the intake manifold removed from the vehicle.