How do you replace a thermostate on a 1998 Bonneville?

First, drain off the radiator fluid to a level below the upper radiator hose. Then disconnect the hose from the water neck on the engine block (and from the radiator if you wish). Remove the water neck, and the thermostat is right there to replace (under the water neck on the engine intake). After replacing the thermostat and bolting down the water neck, reconnect the upper radiator hose, etc, warm the car up. add radiator fluid, then be sure to loosen the screw on the water neck that allows any trapped air in the water neck to escape. You should see a stream of radiator fluid exiting the vent on the water neck. Retighten the vent screw, and fill the radiator to the top and replace the pressure cap, and you should be good for a long time.