Remove the plug on the lower rear of the drivers side on the engine block. If it doesn,t drain poke something in till it does. Flush it real good with water till it runs clear. Pull the radiator & take it to a radiator shop to have it cleaned & tested or buy a new one. Chances are 10 to one 1 it will need a new one or recored. Put a 180 degree therm. & a 4-7 lb. pressure cap.
