The steps is quite straight forward :
1. In the Finder navigate to Applications -> Utilities -> Terminal and open the Terminal app.
2. Enter this command line by line
cd ~/Library/Application\ Support/MobileSync mv Backup old-itunes-backup mkdir /Volumes/pingo/ios-backup/Backup ln -s /Volumes/pingo/ios-backup/Backup ~/Library/Application\ Support/MobileSync/Backup ls -l mv old-itunes-backup /Volumes/pingo/ios-backup/.
*pingo/ios-backup is your new own directory path. Reference : https://www.iphonefaq.org/archives/975865