If you have units you no longer need and want to free up some capacity for new units, you can scrap a unit from the "Scrap/Dismiss" tab in any of the buildings that produce combat units (Barracks, War Factory, or Airfield). You will get back some of the resources you put into the unit, but that quantity will be much smaller than what you've invested in the unit, so don't scrap it unless you're really sure you want to do so!
