Apple allows you to only install iOS firmware that they ‘sign’. After they release a new version of iOS they quit signing the previous version. It’s because of they way Apple uses this secure signature system that prevents people from downgrading there iOS firmware.
Many people believe that to go back to a previous version of iOS software would require them to jailbreak their device. That simply is not true. Jailbreaking makes it easier to downgrade your iOS software because Cydia automatically saves your SHSH blobs for you but you can save your SHSH blobs without jailbreaking.
There are plenty of reasons why someone might want to go back to previous version of iOS. Several people complained about iOS 5 having battery draining issues, not having multitasking gestures, the revamped music app and some people stated they would have preferred to stay on the previous version of iOS.
SHSH blobs are a unique key for your device, this is what Apple checks in their ‘signing’ process. If you have not saved your SHSH blobs you can not downgrade your software.
It’s a good idea for everyone to save their SHSH blobs wether you plan on jailbreaking your device or not. So even if you can’t downgrade right now because you haven’t saved your SHSH blobs, you can save them so you can downgrade in the future.
How to Save Your SHSH Blobs
I recommend using TinyUmbrella to save your SHSH blobs. TinyUmbrella is an application that works on both a Mac and PC that not only saves your SHSH blobs for you locally, but it will trick iTunes into allowing Apple to sign your firmware so you can downgrade to a previous version.
How to Restore to Previous Firmware
After you have saved your SHSH blobs it’s fairly easy to restore to a previous firmware but remember you can only restore to firmware that you have saved your SHSH blobs for, unless you have jailbroken the device (Cydia automatically saves and stores them for you.) Click here to find out how to retrieve SHSH blobs from Cydia.