System/Library/PrivateFrameworks/amework/Versions/Current/Resources Reinstall iTunes while the phone is connected: Updating to iOS 14? This thread discusses a trick to enable iTunes syncing to work in El Capitan.
Note, some downloads from Apple do not work in Safari for earlier Mac OSes, see this tip if you find you can't download them anymore, to find a browser that will work. This tip is designed to be a one stop shop to find out what iOS you can use if you are able to install a specific Mac OS X.
