You could take some water out and mix more salt in, then put it back after it thoroughly mixes. Or, like yote said, top off whatever evaporates with salt water.
I concur with the above postings just do it a little at a time and remeber if you have too high of a SG the pull some out and replace it with fresh, or top off with fresh either way hope this helps.
P.s. remember your water evaporates the salt does not so do not top off with salt water unless raising your salinity. :)