If you are just adding all you have to do is add RO
water to your tank... The salt shouldn't really evaporate so adding more will just increase the levels. If you are talking chaning then that is kinda a different thing. In that case just get a gravel vac (could just use a syphon but kill two birds with one stone) and vac the bed until you are about 10-20% down. Before you do that you have to have the other water prepared though. It is best to let it aerate first. Some people put RO
water into a bucket... fix the salt mixture to the desired level... and run a powerhead to help with aeration. Also you can (are supposed to) heat the water before hand but honestly if it is a small change I usually don't. You just can't flex the temp too much cause it is stressful for the fish. I am not sure if I am forgetting anything. I actually bought a brand new (obviously has to be) gasoline container that holds six gallons of water. Obviously that is a bit more than 10% of a 55 gallon so I do a change about once a week since it isn't much. Makes pouring much easier because of the spout. I was sick of spilling the bucket on myself
. Also easier to store away than a bucket because it is taller and thinner.