iPhone-Dev Team has released RedSn0w 0.9.11b1 which enables you to downgrade on A5 iOS devices such as iPhone 4S, iPad 2 and New iPad (iPad 3).
To downgrade to a lower firmware for iOS devices is possible only when you have saved SHSH blobs.The latest version of Redsn0w enables you to restore to an older firmware without using iTunes.Download the latest version of Redsn0w, Navigate to Extras >>Even More>> Restore to restore to an older iOS firmware.
According to iPhone-Dev Team blog-
The new feature is at Extras->Even More->Restore
You cannot downgrade without the personalized SHSH blobs for your device at that lower firmware. You need to have fetched those blobs while the signing window was open, using either Cydia’s built-in TSS@Home feature, or with TinyUmbrella. The new Restore screen of redsn0w lets you choose either the remote blobs or local ones (for the earlier firmware). If you don’t know where TinyUmbrella put your blobs, TinyUmbrella has a button that will show you (copy them out of that folder and feed them to redsn0w).
The A5 downgrade method actually updates to the latest firmware before downgrading to the earlier one.This process updates your baseband to whatever is newest.DO NOT USE THIS METHOD IF YOU RELY ON UNOFFICIAL UNLOCKS of your iPhone4S. Those who used the temporary SAM technique to unlock their iPhones to specific SIMs shouldn’t be affected by this baseband update.
This method can be fixed by Apple with a firmware update. It’s a (pleasant) mystery why they haven’t fixed it yet, because reverse-engineering of the restore ramdisk indicates they do know about it. It’s possibly too niche to bother to fix right now.
The least-tested devices with this method are the iPad2,3 and iPad3,2 (because we don’t have those models). If you do and you feel like experimenting, please let us know how it turns out in the comment section below!
This update involves a bunch of new redsn0w code. We recommend sticking to the previous version 0.9.10b8b unless you’re specifically using this new feature, until all the bugs are worked out! (Note: If redsn0w gets stuck at the “Waiting for device” stage for more than 30 seconds, you’ve hit a pesky GUI bug…that will be fixed in an upcoming version!)
Download Redsn0w 0.9.11b1