A partial water change is completed by syphoning out water and detritus or anything else that can be releasing toxins, and replacing that quantity of water with the same amount of fresh saltwater. Most people let the new batch of saltwater aerate overnight.
During your cycle test frequently. Ammonia feeds the bacteria that will convert ammonia into nitrite. Then bacteria will form that feed off of nitrite. You should see a slight drop in ammonia and a rise in nitrites at this point. Then the bacteria that feeds off of nitrate will convert it into nitrates. You will then see a drop in both ammonia and nitrate, and a spike in nitrate. Then and only then should a pwc
be performed. There isn't a bacteria that feed off of nitrate but algae sure as heck do. Pwcs help keep nitrates at low to undetectable levels.