Top up if needed

The target volume for this batch of beer is 5 gallons (19 liters). If you end up with slightly more than that, don't worry about it. But if you're a little short, add cold tap water to the fermentation vessel until you have the right amount.

Strictly speaking, you should boil and cool any water that you add to your chilled wort. But many homebrewers do just fine with straight tap water.