"If you updated to iOS 7 from iOS 6, and are now suffering from motion sickness or acute day-glowitis, I have bad news: You can no longer downgrade from iOS 7 to iOS 6. You’re stuck. This is partly for security reasons, but mostly because it’s simply part of the Apple experience: When buying an iPhone or iPad, you implicitly sign away your rights and privileges so that everyone can coexist in blissful unfragmented harmony in Apple’s sandbox.
Whenever Apple releases a new version of iOS, there is a grace period – usually of a few days – where you can revert back to the previous version. There are also ways to downgrade if you happen to have an iPhone 4, which was the last iPhone to be properly and irrevocably jailbroken by hackers. If you have an iPhone 4S, iPhone 5, or iPhone 5S, however, where Apple’s low-level security mechanisms are still intact, there’s no way to revert back to iOS 6 once you’ve installed iOS 7 and the grace period has passed.
To understand why you can’t downgrade to iOS 6, we need to look at the iOS update and restore processes. Basically, after you install a new version of iOS – either over-the-air on your device, or via an IPSW file in iTunes – your device phones home to Apple. Apple then checks to see whether the newly installed version of iOS is signed – as in, the cryptographic signature of the files matches a list of approved signatures on Apple’s servers.
This serves three purposes: Firstly, it makes sure that none of your files are corrupted or infected with malware. Secondly, it ensures that you’re only running an approved version of iOS – and thirdly, there’s the small issue of fragmentation. One of the strongest features of Apple’s mobile ecosystem is that every device runs the same operating system. If users could downgrade freely, then the story might be very different."