It's very possible to cause engine damage. You need to drain out half the water by slackening a radiator hose & then replacing this with anti freeze. A Haynes manual will tell you the total coolant capacity - Either just let out at least half & then refill with half the total volume of anti freeze, topping up with water. Remember to keep a bit of anti freeze for the expansion or header tank.
Or, drain as completely as possible & premix anti freeze as per the container & refill from scratch.