Quite a bit of useful information, thanks! I have an iPhone 4 running on 5.0.1, I want to do the opposite and upgrade to 5.1.1b and I have saved blobs on Cydia for 5.0.1 upto 5.1.1b, I verified this with TinyUmbrella.
How do I take the next step and upgrade? Will I have to rejailbreak with redsn0w?
Since you can't do a simple upgrade in iTunes anymore (Apple will force v6 down your throat), you will have to basically follow the same procedure that someone would follow downgrading to 5.1.1b. I'll try to summarize the steps below:
1. Find and download the 5.1.1b firmware
2. Download the latest version of Redsn0w
3. (recommended) Make sure your iPhone runs an iTunes backup. You might also want to check and see if your apps have any kind of backup features within them. For instance, I always run the backup on my Momento app because that is really the only app data I care about, everything syncs to cloud services so not so important if my backup doesn't work quite right...
4. Load up Redsn0w and goto (this is from memory) Extras->SHSH stuff->Stitch SHSH to IPSW.
5. Browse to the 5.1.1b .ipsw firmware and select it
6. Under the SHSH blob option choose the "Cydia" option (note: your phone will need to be plugged in via USB for this option to be available)
7. Wait for the new .ipsw file to be created (it takes a few minutes). It will be saved with a new file name that has your device UDID as the prefix.
8. Place your iPhone in DFU mode. There is even an option in RedSn0w to boot to DFU mode for custom IPSW or something. Be sure to put your iPhone in DFU mode and not Recovery mode (iTunes refers to them both as Recovery Mode but there is a difference). An example of what NOT TO DO (because I did this): if you were to try and load the new custom firmware in iTunes w/o your iPhone being in DFU mode, iTunes would attempt place your iPhone in Recovery mode attempt to load some stuff and then throw you an error with your iPhone stuck in Recovery mode. For some reason once you are in Recovery mode it is difficult to get the device back into DFU mode following the normal procedure...... just start in DFU mode and you will be fine. DFU mode is the one where you have to hold the power down for 3 seconds, then while holding the power press the home button and hold for 10 seconds, then release the power button but continue holding the home button for like 5 seconds or w/e (follow real directions like the ones in Redsn0w, not my directions here).
9. iTunes will detect that an iPhone is connected in DFU mode (it will call it "Recovery mode") and ask you if you want to restore to the latest version, cancel out of that dialog box.
10. On the iPhone page, hold down Option (I think it is ALT on Windows) and click "Restore". A dialog box will open up for you to select your SHSH blob stitched firmware file.
11. Wait for iTunes to finish its stuff. It will end with some Unknown -1 error. That is to be expected. If you were paying attention you would notice that the progress bar on the iPhone goes to complete and then this unknown error pops up. It takes time to load the firmware. If you immediately get an error then you probably did something wrong.
12. Click through the error. You will notice that your iPhone boots into a recovery mode (it has the cable to iTunes screen on it).
13. Goto Redsn0w and under the Extras select "Recovery Mode Fix" (or something like that). You need to follow the steps to kick it back into DFU mode. It runs a bunch of stuff on the iPhone that looks like it is jailbreaking it but it is not (it is just fixing it so it isn't in recovery mode). When this is done, you should be able to boot and be in 5.1.1.
14. Restore your iTunes backup
15. Re-jailbreak your iPhone using Redsn0w
16. Re-install and configure your jailbroken apps from Cydia.