As always, the devil is in the details. It depends why your battery is low. If it is not sealed, and there is evaporation or heat, it may lose some water. In that case, adding back can help.
But AGM shouldn't have this problem, and it should be sealed well enough to avoid evaporation.
Also it depends how badly gone the acid is, and how much exposure to the air there has been. If the acid has eaten away the inside of the battery, adding water will not help and may make it worse.
Generally speaking - don't fix what isn't broken. Always consult the manufacturer for specifics like this and follow their recommendation. YouTube can be a dangerous place to be if you don't know what you're doing.