Evaporation happens continuously, many new hobbyists start by adding Freshly mixed saltwater then end up with a really high salinity. You have to think of your water as two elements, in fact it is. Salt and Reverse osmosis water. Salt cannot evaporate because it is a solid and solids don't evaporate. Therefore as the water evaporates, the salt remains in the Aquarium, which in turn increases your salinity. From this you should have worked out that Top up/RO water should be added.
If your Aquarium is left for a few days or more the salinity will rise rapidly, which is why it is important to top up daily if possible.
When carrying out a water change, because you are removing the mixed salt water your are removing both elements, salt and RO water. All that is required now is a fresh saltwater mix that has the correct salinity of about 1.025.
Hopefully this as helped you grasp this simple regime. This can now be taken a little further. Maybe your salinity is high or low, this again can be adjusted in just a simple method.
If it is high, just remove a few liters of saltwater and add some fresh RO water.
If it is low, just add the correct salinity water to the aquarium instead of RO water, Only a few liters at a time though then check Salinity until desired is reached.
An even better way to top up RO water is with the use of an auto top up unit. Like the inexpensive TMC V2 auto top up unit that simply automates the process and gives you stability that the reef aquarium needs for improved growth and coloration of corals.