We can utilize of what Redsn0w 0.9.10b5's offer to downgrade to the older iOS firmware versions as long as the SHSH Blobs of the said iOS versions saved. Because that's the unquestionable requirement, without it there is no way to downgrade back to the older iOS versions.
And this method can be applied to older devices like iPhone 3GS, iPhone 4, iPad 1 or iPod Touch. For A5 device users, currently there is no available workaround to downgrade your iOS 5.1 even if you saved SHSH Blobs. Okey now let's downgrade back to the iOS versions of your choice.
How To Downgrade iOS 5.1 To iOS 5.0.1, 4.3.5, 4.3.4, iOS 4.3.3 On iPhone, 4, 3GS, iPad and iPod TouchRequirements:
Download Redsn0w 0.9.10b5 based on your existing Operating System.
Step 1: Open Redsn0w.
Step 2: Choose Extras option, and then choose Pwned DFU option.
Step 3: Plug your device to your PC, continue by putting your iPhone into DFU mode, use the the on-screen instructions to do so.
Step 4: Now point iTunes to connect to Cydia server, and not to Apple server, to do so let's modify the host file. If your Windows users, use your Notepad. And As for Mac, use your Textedit. Then go to this path.
Windows Path : C:\Windows\System32\drivers\etc\
Mac Path : /etc/hosts
Step 5: Put the following line at the end of the entire file, save it, afterward (see the picture).
How To Semi Untethered Jailbreak iOS 5.1 On iPhone 4, 3GS, iPad 1 And iPod touch.