Depending on if you have an ATO that you need to work around or not then you either do or don't need to take water out. In my setup to keep the water level in my return area of my sump where I want it I have to remove water and add fresh to fix high salinity. You could just add fresh water until it's right. Slowly of course.
I was hoping you'd say a refractometer. But be sure that you are waiting long enough for it to temperature adjust. If you had it stored then when you take it out to use it place a water sample on it and wait a minute before looking at the reading. You may or may not get a different reading. Also get some calibration fluid for your refractometer too to make sure it's where it needs to be.
Sent from my iPhone using Aquarium Advice