Following the release of iOS 10.3.1 on April 3, Apple has stopped signing iOS 10.2.1 and iOS 10.3, earlier versions of iOS that were previously available to consumers before the latest update was released.
Customers who have upgraded to iOS 10.3.1 will no longer be able to downgrade their devices to iOS 10.2.1 or iOS 10.3.
Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to stay up to date.
iOS 10.3.1 are now the only versions of iOS 10 that can be installed on iOS devices by the general public, but developers and public beta testers can also download iOS 10.3.2, a future update that is currently being beta tested.