Q. I sometime stand up for a fifth rakat. Would I just sit down after the completion of the fifth rakat, or do I make a sajdah sawh after it?

A. A person standing up for the fifth rakat later remembers that an access in rakats has been created, all they have to do is merely sit down prior to the sajdah of the fifth rakat. If fifth rakat was read (including its sajdah), than merely make it a 6th rakat salah. In doing do, the entire salah would become an act nafl. At this point, merely repeat the fard salaah again, with its appropriate rakats.

Allah Certainly Knows Best.