simply drain out radiator liquid when engine runnnig ,after liquid drained, close drain plug add factory recomended coolant till no air bubble in radiator , let engne cool down, preferblly overnight, chk and add coolant to top up before starting car next day.
water has too less boiling point at 100c so it will turn into steam inside engine, use coolant. or you will be changing engine headgasket pretty soon too from overheat suddenly.