Are you using Windows or Mac?
For windows:
Firstly make sure you show hidden files - follow the simple instructions on this link:
http://windows.microsoft.com/en-gb/windows-vista/show-hidden-files
Backups will be at the locations below (choose your system)
Windows XP:
\Documents and Settings\USERNAME\Application Data\Apple Computer\MobileSync\Backup\
Windows Vista or Windows 7/8:
\Users\USERNAME\AppData\Roaming\Apple Computer\MobileSync\Backup\
Replace USERNAME with your personal account name.
Make a copy of the backup located in the Backup folder (individual backups are shown as folders, each with a long string of numbers as a title) - to your desktop. There might be more than one backup in this folder, so choose the correct one - use the time made to decide which one is which. This copy needs to be put back in the above location in the future if iTunes copies over it...it is safe on the desktop from iTunes deletion.
For Mac -
~user/Library/Application Support/MobileSync/Backup
Repeat the same procedure as windows.