Welcome to the forum!
I'm assuming you have a saltwater tank?
The water should be filtered either through a RO
filter, or if you don't have one you could add dechlorinator to the water.
To replace the water in the tank, you need to mix the salt and water in a bucket first (ideally with a powerhead for 24 hours or so) with a heater, so its temperature is as close to your tank as possible and then if you have a powerhead use it to pump water back in.
If you don't have a powerhead, you could have someone help you take the bucket and slowly add water back to the tank.
Theres no real science to it but you want to make sure the water/salt is mixed as well as possible, and the new water is as close as possible to your tank water in terms of temperature, salinity, etc.