how hard is it to replace the thermostat on the 02 couger v6

It's a very easy operation. Put a drain pan under the "front" of the engine and disconnect the coolant hose from the water neck. Try to keep the hose facing up while you are clamping it shut to prevent coolant loss. Next, undo the bolts securing the water neck, then pull out the old thermostat and put in the new one. Make sure you install any O-rings or seals. I would also recommend replacing the water neck gasket. Then, put everything back together and replace any lost coolant. If your coolant is discolored, you should completely drain the system, flush the radiator, and put new coolant in. The radiator can be flushed by disconnecting both hoses, draining them into the drain pan, and running clean water from a hose through the top hose connection until the water runs clear out of the bottom.