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Recently, many news sites have reported that Apple does not allow iPhones and iPads to downgrade to iOS 9.3.5, which means that we can no longer proceed with iOS 10 downgrade to iOS 9 for all current iPhone, iPad, iPad devices when we have upgraded iOS 10 for our devices.
As of this morning, Apple has officially stopped licensing the last upgraded version of iOS 9, iOS 9.3.5, for iPhone, iPad, iPod devices, so we can also understand iOS 9 is officially dead and users can no longer downgrade from iOS 10 to iOS 9 for devices
In addition, Apps has officially stopped licensing for the first iOS 10 upgrade version, iOS 10.0.1, which also means that users can no longer downgrade iOS 10.0.2 to iOS 10.0.1 and from now on can only downgrade to iOS 10.0.2.
https://Software/apple-khong-cho-phep-iphone-ipad-ha-cap-xuong-ios-935-6791n.aspx
If you still do not have time to upgrade iOS 10.0.2 for your device, you can refer to the article how to upgrade