Following the release of iOS 16.1 on October 24, Apple has stopped signing iOS 16.0.3, the previously available version of iOS released on October 10. Since iOS 16.0.3 is no longer signed, it is no longer possible to downgrade to that version of iOS after installing iOS 16.1.
Apple typically stops signing software updates for older versions after a new version is released to encourage customers to keep their operating system up to date, so it’s not uncommon to no longer sign iOS 16.0.3.
iOS 16.0.3 is a minor bug-fixing update that addresses a bug plaguing the iPhone 14 and iPhone 14 Pro models.
The iOS 16.1 update that replaces it is even bigger, introducing iCloud Shared Photo Library, real-time activity on the lock screen and Dynamic Island, support for the Matter smart home standard, clean energy charging, and more, see our iOS 16.1 guide for details.
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Hazel Yu 
Ken Cheng 