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It's not possible to revert to iOS 6. Apple made sure that the only software you'll be able to install on an iDevice is the one signed by them. At this very moment, these are 9.3 and 9.3.1.
While updating, iTunes connects to Apple servers, in order to verify the software you're trying to install. If it's a different one than those signed by Apple, you'll get an error, making it impossible to proceed.