How far back was the back up...
iOS does not support restoring a backup from a newer version of iOS to a device using an older version of iOS. Example: Restoring a backup from an iOS 7 device to an iOS 6.x or earlier device is not supported. If you wish to transfer content from a device with a newer version of iOS to a device using an earlier version, follow these steps to transfer the content to your computer, and then use iTunes to sync the content to your device. Note: Some content, including messages, app data, and other settings, cannot be transferred using this method.