Gravity points are conserved
Adding or removing water does not change how much sugar is in the kettle, only how concentrated it is. So the total gravity points multiplied by volume stays constant, and you can solve for whichever you need.
A positive result is water to add. A negative one is how much you need to evaporate.
Worked example
18 litres sitting at 1.064 when you wanted 1.050:
What else moves when you do this
Diluting drops everything, not just gravity. Bitterness and colour fall in the same proportion, so 5 litres of water into an 18 litre batch takes roughly 22% off your IBU and your SRM as well. If the recipe mattered, recalculate those too.
Boiling down does the reverse and is worse, because it is not proportional. Concentrating the wort raises boil gravity, which lowers hop utilisation, and extends the time at temperature, which darkens the wort through Maillard reactions. An extra 20 minutes of boil to hit a gravity target will cost you bitterness and add colour beyond what the volume change alone predicts.
Adding water
Use water you would be willing to drink, and get it to roughly the temperature of the wort before it goes in. Cold tap water into hot wort is a thermal shock to nothing in particular, but cold untreated water into cooled wort is a contamination route. If you are topping up after the boil, boil the addition first and cool it.