If I recall, you take nut off the other side of arm. Press assembly out. At that point sliding gear slips off threaded end of shaft. Ball detent spring inside, as well, but not super high sprung, just watch for pieces as it comes apart.
Prior to assembly. Orient arm to how it sits on machine. Then look at oil hole in shaft. Orient that hole so its at 12 oclock, so oil can run down hill in shaft. Same with other shafts/gears on banjo.