Thermostat change - 1999 Audi A6

You're going to lose some coolant, so have a bucket or other container ready. Follow the upper radiator hose to the point where it's connected to the engine. The hose is clamped to a metal housing which is bolted to the engine. Remove the bolts (not the hose) and lift the housing to expose the thermostat (may require a little prying with a screwdriver or similar tool). Note how the old thermostat is fitted (which end is up), lift it out of the engine, and then remove all traces of gasket from the housing and the engine. Install the new thermostat and gasket, replace the bolts, add coolant as necessary, and you're done.