Launch Terminal from your Applications:Utilities folder.
Navigate to the MobileSync directory by inputting the following command and pressing Return.
cd ~/Library/Application Support/MobileSync
Rename your old backup folder using the following command.
mv Backup Backup_Old
If you haven't created a new folder to store your backups please do so now. You can do this with Terminal or Finder, or you can use an existing folder.
Example: mkdir /Volumes/Storage/iTunes/MobileSync/Backup
Input the following command to create a symbolic link to your new folder.
ln -s /Volumes/Storage/iTunes/MobileSync/Backup ~/Library/Application\\ Support/MobileSync/Backup
You can use Finder to verify the symbolic link was created. Notice the arrow at the bottom left of the icon.
Test out the change by performing a backup with iTunes and verifying that your files are backed up to the new folder.