What I did was top off with saltwater till it was at the lvl I wanted. Don't just add straight up salt to the tank though, its not good for anything you would have living in the tank. You could also do water changes with a higher salinity than the tank is alrdy. --- If you have nothing in the tank though, you can just add salt!
To raise your salinity insead of toping off with fresh just add salt water untill your salinity is at a disirable level then jsut go back to topping of with fresh.
To raise my salinity instead of toping off with fresh I just top off with saltwater untill my salinity is where I want it.
But if you do this check your salinity offten to make shure you don't over do it.