in a 58 gallon, once it is running, I would do no more than 10%every week, if it is well established, and going good, 5% would be good enough
just siphon(pythons work good, just don't get it down in the sand) out an equal amount or scoop out with a clean unused(not washed because of the detergents in soap!) pitcher, then replace with dechlorinated saltwater, best to circulate in a bucket(again clean and unused)for a couple of days with an airstone or powerhead first