Can I downgrade my iPhone firmware from iOS 6 to iOS 5.1.1?
Apple has stopped signing firmware older than iOS 6. This means that downgrading to an earlier version of iOS requires SHSH blobs and custom IPSW files.
Luckily for iPhone 4 and iPhone 3GS owners who have saved their SHSH blobs, there is a procedure that will downgrade your device to iOS 5.1.1. Full instructions on how to downgrade from iOS 6 can be found here.
For those iOS users who have non-A4 devices, you are currently stuck with iOS 6. Only A4 devices can be downgraded, including the iPhone 4, iPhone 3GS and iPod touch 4G. If your SHSH blobs for iOS 5.1.1 have not been saved to your computer or Cydia, you are also stuck with iOS 6.