Raising Salinity

[Beanz]

Hey guys, as the title states I have low salinity in my 15 gallon tank, total volume of 25 gallons (that's with sand and rock accounted for.) It is at 1.021 and i wanna raise it to around 1.025-1.026. What would be the best method small water changes? or put a small amount of salt in my ATO water? I just wanna do it slowly so i don't stress my livestock.