How do you replace the thermostat on a 2000 GM jimmy?

Follow the upper radiator hose down to the engine. It attaches to the thermostat housing. Two bolts hold the housing on. There may be accessories, brackets, etc, that may have to be removed because they will be in the way. Remove the housing, lift out the old thermostat, remembering which way it came out, and replace with a new one. Newer vehicles now use seals instead of gaskets. Use no sealer when putting the housing back on.