Increase it slowly! The easiest way is to just add saltwater instead of fresh, for your top-offs. This way too, it is a gradual change.
Ohh, slowly only applies, if you have some stock already in the tank. If no stock then, take some water out, mix in some more salt, and add back to the tank. It might take a couple times, but a little at a time is best, so you don't go over with the salinity.