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You Can No Longer Downgrade From iOS 27 to iOS 26
Apple is no longer signing iOS 26.6.2, so if you've upgraded to iOS 27, there is no option to downgrade to a version of iOS 26. iOS 27 came out a week ago. Software that is "signed" can pass a server-side verification check that Apple performs when a new version of iOS is downloaded. If an update fails the verification check, it can't be installed. By unsigning software, Apple prevents customers from installing outdated, less secure versions of iOS.
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