Following the release of iOS 10.2.1 on January 23, Apple has stopped seeding iOS 10.2, the previous version of iOS that was available to consumers.
Customers who have upgraded to iOS 10.2.1 will no longer be able to downgrade their devices to iOS 10.2.
Apple routinely stops signing older versions of software updates after new releases come out to encourage customers to stay up to date. iOS 10.2.1 is now the only version 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, a future update that is currently being beta tested.
Source:: Mac Rumors