The recently introduced iOS7 has met up to the expectations of the majority of Apple users. However, there are still some who for whatever reason, aren't happy with iOS 7 and want to downgrade to iOS 6. Some have shared the set of instructions for getting back the original beta, but now, those who are on the public release of iOS 7 can no longer downgrade to iOS 6.
The instructions offered by many reputable sources were for users that were running the beta version of iOS 7 before it was ever made available to the public. Almost immediately after releasing released iOS 7 to everyone, Apple stopped signing iOS 6 installs, meaning that if you try and downgrade back to iOS 6, iTunes will produce an error message saying that you can't. This causes a big confusion among users, as take the error message as if they did something wrong. However, it simply means you're running a newer version, aka iOS 7, and Apple doesn't want you to go to an older version.
This is a common practice of Apple, used usually after releasing a new version of iOS. Unfortunately there is no certified way to get back to iOS 6 by now, but as soon as we have one, we’ll definitely share it with you.
And finally, for those who already tried downgrading back to iOS 6 from iOS 7 and got error messages, simply restore your iPhone back to iOS 7 to get everything back up and running again.