Water can be added. This will dilute the wort you have boiled by 20%. Specific gravity will be lowered. Current hop bittering and flavor will also be reduced by the addition.
Is the post boil gravity much higher than the estimated OG of the recipe? How does the wort taste? Go with the addition if the OG is higher than estimated in the recipe and the dilution of the hop bitterness/flavor won't give you just a very weak tasting beer when bottle conditioned.
I would let it go as is if the wort tastes good even if it will be a little higher in ABV.