I have a 1992 civic and when I turn the heater on it blow warm air it all the way in the hot position but it only blow warn air and the engine temperature gage is at regular operating temperaturn I check all the fues and all were good I was thinking maybe it need a coolent flush ? Any idea

On a vehicle that old, you could have sediment in the radiator, or the heater core, or both. The condition of the hoses is suspect as well.
Are both heater inlet and outlet hoses getting hot? If only one is warm, the heater core may need replacing.

Which one is the inlet and outlet house where is it connected to how do I find it