Why can't I restore my iPhone 5 from backup after IOS 9 update?

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cannot restore iPone 5 from backup after IOS 9 update

Upgraded to IOS 9. Passcode required but completely unable to remember the passcode. Had to reset iPhone but now IOS 9 incompatible with old iTunes version to restore previous backup. The only thing needed is the pictures. Cannot upgrade iTunes because of error code 2721. It has been suggested that the current iTunes be uninstalled but the backup will be destroyed.