How to Update iPhone 3GS / 3G Baseband to 06.15.00 with Redsn0w 0.9.6b5 for iOS 4.2.1 Unlock

Redsn0w 0.9.6b5 has just been released which can jailbreak and update iPhone 3GS & 3G baseband to 06.15.00 so that it can be unlocked using the newly released Ultrasn0w 1.2.

The new redsn0w 0.9.6beta5 is out.  It gives both Windows and OSX users the ability to flash the iPad 06.15 baseband on iPhone3G or iPhone3GS.  It fetches the baseband files

directly from Apple for now (the only IPSW you ever point it at is the stock IPSW for the FW on your iPhone right now).  There may be a long delay while it’s doing this (their servers are getting pounded right now)
If you do flash your baseband via redsn0w, please keep it plugged into USB the whole time.  You don’t want your battery to die during the flash process!

WARNING:

YOU DO THIS AT YOUR OWN RISK!  PLEASE UNDERSTAND THE CONSEQUENCES OF UPDATING TO 06.15.

1. There is no way to come back down from 06.15, and there’s no hiding the baseband version from Apple.  You’ll be voiding your warranty in a very obvious way.
2. 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).
3. Starting with FW 4.2.1 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).

Note: This release of Redsn0w is mainly targeted for iPhone 3GS & 3G users on iOS 4.1 / 4.2.1 who rely on a carrier unlock. All iPhone 4, 3GS, iPad and iPod touch users who don’t rely on a carrier unlock should follow the instructions posted here to jailbreak their iOS device on iOS 4.2.1 using Redsn0w 0.9.6b4.
Follow the instructions posted below to update your iPhone 3GS or 3G to 06.15.00 baseband and jailbreak it on iOS 4.1 /4.2.1.
Step 1: Download iOS 4.2.1 for iPhone 3GS or 3G.
Step 2: Update/Restore to iOS 4.2.1 firmware using the IPSW file you just downloaded via iTunes 10.1.
Step 3: Start Redsn0w 0.9.6b5 and point it to iOS 4.2.1 firmware to proceed with the jailbreak.

 
Step 4: Now select “Install Cydia” & “Install iPad baseband” options and then click “Next” to continue. Redsn0w will now fetch the exploitable 06.15.00 baseband from 3.2.2 firmware and will replace it the baseband in iOS 4.2.1 / 4.1.

Note: Install iPad baseband option is applicable to iPhone 3GS & 3G users only who rely on a carrier unlock.
Step 5: Now make sure your device is both OFF and PLUGGED IN to the computer before you click “Next”.

At this stage you will be presented with a screen where you will have to quickly hold “Power” button and then while holding the “Power” button, you will now have to hold “Home” button too. Now after few seconds release “Power” button but keep holding “Home” button until installation begins

Step 6: Sit back and enjoy while Redsn0w will do all the rest for you. Once done, your iPhone or iPod touch will restart automatically and you will find it jailbroken on the latest iOS 4.2.1 firmware, with the unlockable 06.15.00 baseband.
For iPhone 4, 3GS (New Bootrom), iPod touch 4G / 3G / 2G, iPad ONLY:
Step 7: Now from here onwards, whenever you want to do anything related to jailbreak like SSHing into your device, or running Cydia, you must first run your device in the so called “jailbroken state” on every reboot by using “Just boot tethered right now” option in Redsn0w.