To change the actual port used by Resilio Sync you have to edit the sync.conf file in '/volume1/@appstore/resiliosync/var'. Assuming you are installed on volume 1. You'll see a section with 'listen":"0.0.0.0:xxxxx', change whatever the x's are to the port you want.
Not 100% positive on changing the link that Syno shows in the package tab but I think it is in '/volume1/@appstore/resiliosync/app/config'. However, I've changed mine and it did not update in the Syno gui, but I have not rebooted which may be required.