Apple has stopped signing iOS 8.4 for compatible iPhone, iPad and iPod touch models, meaning that users can no longer upgrade or downgrade to that version using iTunes. Apple is now signing iOS 8.4.1 and later only.
iPhone, iPad or iPod touch users that have jailbroken their device on iOS 8.4 should refrain from updating to iOS 8.4.1, as the latest software update breaks the iOS 8.4 untethered ...