Go to the dealer and tell the parts guy that you want to change your coolant. He will give you two bottles. Use them both. If you get straight coolant you will need to cut it 50/50 with purified water. The other bottle is a conditioner that Subaru recommends.
Only dedicated track cars should use only water. It will not cause the car to overheat. Instead it will reduce engine temps better then coolant water mix. The danger is it could freeze and it could cause rust.