This process is only possible using a downgrade, which can be a fairly difficult operation to complete (Apple authenticates all new software installs using their own servers).
SHSH blobs are in the process of being phased out in favour of the tougher-to-beat APTicket.
More to the point, most i OS software updates modify your hardware’s baseband version, which means if you’re dependent on your jailbreak for an unlock, you will lose your unlock when you finally do upgrade.
One way of potentially restoring your unlock is to save your SHSH blobs, which is what is used to authenticate your i Phone with Apple.
For most, that option simply won’t work and others have reported experiencing battery life problems and activation issues when updating on top of a jailbreak.
Important: For this reason, you shouldn’t upgrade the software on your phone or restoring to stock if you depend on your jailbreak for an unlock.
We recently covered the release of Luca Todesco’s first build of yalu102.
This video will show you how to restore you i Pod touch or i Phone to the original version.
This will erase everything off you device and return it to the factory settings.