Where is thermostat located on 1999 Acura 2.3CL

The top radiator hose will lead you to the thermostat housing on the engine block. The thermostat will be located inside the housing. Likely, you only need to remove two 10 mm bolts, after you drain the coolant to a level below the thermostat. Be sure to install the new thermostat in exactly the same position as the old one.