The instructions at What to do before you install iOS beta - Apple Support for archiving a device backup mention control-clicking on the backup in iTunes -> Preferences -> Devices which displays a popup menu, but this doesn't work on a PC.
PC-OTHER, Windows 8
It doesn't. The existing backup set associated with the device is renamed and updated. No new backup set is created.
tt2
This is NOT accurate. This suggestion is not useful at all. Follow the answer below locating the backup file and renaming it.
As it happens my current advice on this topic is now found in Archive iPhone backup in Windows which is to copy the backup set and rename it. That said the original approach works and simply reflects what happens if you restore a backup. A manually duplicated backup set won't show with the padlock that iTunes puts on it when it creates the archive.
tt2
How do you archive an iOS device backup on a PC?
