Follow the wide diameter top radiator hose back to the engine - it should end up at the thermostat housing.  Some are a bit awkward to get at.  Drain the coolant, make sure you have a new gasket, clean all traces of the old one off and refill with the right mix of the correct anti freeze and water (vital to have antifreeze, not just water otherwise the head will corrode internally - and the type of anti freeze is critical on many VW group engines, so check the handbook or check with a dealer to make sure you have the right one)

Tim