I agree with everyone here saying to leave it alone and be happy with the higher gravity beer. But, if you really want to add water to cut it down a little, do it earlier, rather than later. Boil the amount of water needed for about 15 minutes, cool, and pour into the fermenter (primary, preferably). Any added O2 during the pouring will help build up the yeast. When doing it later, the yeast aren't using O2, and any added O2 can cause oxidation. So pouring would not be recommended at that point. Syphoning may work, but why chance ruining your beer. I recommend leaving it alone.