MySelf

Recently I was looking for a method to do this and found nothing.
So I tried and figured a method out.

iTunes updates the firmware(baseband) after updating the system, so we can prevent iTunes from updating baseband after the system is updated.

You need to do the following x steps:

1.backup your 5.x shsh blobs
2.run redsn0w0.9.13dev4, select Extra->Even More->DFU IPSW to make a "DFU IPSW" of 6.0
3.add ".zip" to the file, unzip, delete all file that the name contains "04.12.02" in folder "firmware"
4.create a zip file and rename back to .ipsw
5.boot the device to DFU mode, open iTunes, restore iPhone using the new ipsw
6.you will receive an error during the process, and your phone will enter "Recovery Mode"
7.enter DFU mode again, open redsn0w 0.9.13dev4, click "Select IPSW", select the new IPSW, and click on "Recovery Fix"
8.reboot the phone and you will enter iOS6.

If you use ultrasn0w to unlock or need to put activation ticket, etc. You must jailbreak it and you need to connect the phone every time it reboots.

This a screenshot of my phone after updated to iOS6.


I posted this on a Chinese forum and someone got his phone successfully updated: