The thermostat for the 90-93 accord is on the back left-hand side of the engine where the hose connects from the radiator to the engine, disconnect the hose and clamp it off or move it to the side as not to leak antifreeze (use a rag or container for antifreeze that comes from engine block). Unscrew the 2 screws from the hose adapter and take it off alone with the old thermostat. Replace thermostat as well as rubber gasket.
how do i find the thermostat on my 1990 accord