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 jailbreak released in June by Chinese hacking team TaiG.
Chinese team Pangu demonstrated an iOS 8.4.1 untethered jailbreak (via iPhoneHacks) at the HackPwn2015 security conference last week, but the method has not been publicly released. With the public release of iOS 9 expected in less than a month, it is possible the team may hold off on exposing an exploit to Apple for a minor software update.