Connect your iPhone 3GS to the computer and launch iTunes.
Select your iPhone from the list of devices on the left.In iTunes 11 to activate left panel press Ctrl+S.
Create a backup by right-clicking on iPhone name in Devices (iTunes left pane) and choosing Back Up. In iTunes 11 you can click Back Up Now button.
Once iTunes has finished updating your iPhone to the desired firmware open the Pwnage folder on your desktop and launch the redsn0w application from the redsn0w folder we extracted earlier.
Right click the executable and set it to run as administrator.
Once RedSn0w opens click the Extras button
Now click Select IPSW button.
Navigate to the Pwnage folder on your desktop and select the iOS 6 firmware ipsw. Click the Open button to continue.
Windows with a message that selected firmware is identified will appear. If you see an error - either ipsw was corrupted and you need to download it again, or you have chosen 6.0.1 instead of 6.0.
Redsn0w might ask if you have new or old bootrom version. You can detect your bootrom version using our tutorial.
Answer YES, if you have iPhone 3GS with new bootrom.
Answer NO, if you have iPhone 3GS with old bootrom.
Now click Back, to get to the main menu.
In the main menu click the Jailbreak button.
RedSn0w will now prepare the jailbreak data.
From this window you can select the jailbreak options you would like.
Make sure Install iPad Baseband is selected.
You can update your baseband to the 06.15 version for carrier unlock. If you are on 05.14 or 05.15 or higher you will need to do this to unlock. You do not need to do this for jailbreak only. Please read the following very carefully before deciding whether to do this.
You’ll be voiding your warranty in a very obvious way.
If some future baseband comes out with a critical fix, you won’t be able to update to it if it remains down in the 05.xx sequence (then again, you wouldn’t update to it if you wanted to keep your unlock anyway).
If you have 06.15 on your iPhone you won’t ever be able to restore to stock firmware (it will fail). You’ll need to only restore to custom IPSWs (then again, if you’re unlocker you should already be doing that).
Don’t update to 06.15 if you don’t need to, you can just jailbreak without unlocking using this tutorial! Only update baseband if you need the unlock and you’re stuck on baseband version 05.14 or 05.15, and you’re willing to assume the above risks. You can find out your baseband version in Settings -> General -> About -> Modem Firmware.
If you have read all above and want to update your modem to 06.15 select Install iPad Baseband.
If you select Install iPad Baseband warnings will be shown. Click YES if you are sure and you know what you are doing.
Click the Next button to continue.
Please plug your iPhone into the computer and make sure its OFF then click the Next button
RedSn0w will now guide you through the steps to get into DFU mode.
1. Hold down Power button for 3 seconds.
2. Without releasing Power button, hold down both the Home button and the Power button for 10 seconds.
3. Release the Power button and continue holding the Home button until RedSn0w detects the device (~15 seconds).
If you have problems try again.
iPhone will reboot and redsn0w will start jailbreak process.
RedSn0w will then begin uploading the new RAM Disk and Kernel.
Once this is complete you will be notified that RedSn0w is done.
Now you can perform downgrade from 06.15 to 05.13 using this tutorial. Just select Downgrade From iPad Baseband on Step 6.
Now we recommend to create custom iOS 6.1.2 firmware using Sn0wBreeze and unlock using ultrasn0w. Our step-by-step tutorial is availabel here.