Yeah, it'd probably be best if you looked in the manual to familiarize yourself with performance mode and all the ways you can adjust various parameters for the parts = instruments. Much of it is just adjusting numbers in certain screens. Edit is something different, and is not really what you're looking for.
What you want to adjust is Part Volume; you can either do it manually or use (send) CC 7 to lower the part volume. There's also CC 11, expression, but that depends on how you have a couple of other performance parameters set up. For now, just look at Part Volume for the organ part.
BTW: since you are just getting the XP50 going, you'd probably find a *lot* of good intro help at
http://www.rolandus.com/SUPPORT/DOCS/SUPNOTES.HTM Look toward the bottom for the XP50 files. Good stuff.
Good luck.