How to selectively upgrade/downgrade iOS

[evilsatan]

There are a lot of people asking how to selectively upgrade or downgrade their iOS so I thought I would gather information from other sites into a single sticky to help DF members. This guide should work for everyone using iOS4, when iOS5 is released the SHSH method should be redundant.

There will be two main categories of people, those with their SHSH blobs saved and those without SHSH blobs. If you are already jailbroken then launch Cydia and at the top it should say 'SHSH: iOS' followed by version numbers. If the version of iOS that you want is listed here then you can go to the main guide, if your desired version is not displayed then you will need to try iFaith first.

Make sure you backup your phone before going any further, all methods will reset your phone to factory settings.

Click here if you have your SHSH blobs saved
Click here if you don't have your SHSH blobs saved (only compatible with iPhone 4, iPhone 3GS, iPhone 3G, iPod Touch 4, iPod Touch 3G, iPad 1G, Apple TV 2)
Downgrade iOS 6.0.1 to 5.1.1

Notes:
This guide is work in progress, if you experience any problems please post in this thread and I will update the guide to improve accuracy.
Some users have experienced problems when using iFaith so if anyone following that guide could post their feedback it would be appreciated.