Apple has ceased signing iOS 10.2.1 and iOS 10.3, following the April 3 release of iOS 10.3.1. This means customers who have upgraded their devices to iOS 10.3.1 will no longer be able to downgrade their device to the earlier versions.
This is a routine move on Apple’s part, as the iPhone maker regularly stops signing older versions of its mobile operating system in order to encourage users to keep the latest version of the OS installed on their devices.
This makes iOS 10.3.1 the only version of iOS 10 available for installation on iOS devices by the general public. Developers and public beta testers also have the option of installing iOS 10.3.2, an upcoming update that is currently in beta testing.