How can I add lost acid in my sealed solar battery?

If you’re asking this question I would assume you have a duff battery you are trying to salvage. Measure the voltage under a moderate (20–50 watt) load. If, for a 12 volt battery, it rapidly drops to 10 volts or under you have a collapsed cell and it’s unlikely you’ll recover it.

However, you can prise the top off and gain access to the “valves” then remove the rubber valve caps. You don’t need new acid, just distilled water will do. Not too much or it will flood when charged.

That might help, but probably not. However, if the battery is stuffed anyway you have nothing to lose by trying it.