By using Redsn0w 0.9.6b5 jailbreak tool, iPhone 3GS and iPhone 3G running over iOS 4.2.1 can get update to the baseband 06.15.00, and then those iDevices could be unlocked with Ultrasn0w 1.2.
Following guide can help you to update iPhone 3GS, 3G baseband to 06.15.00 with Redsn0w 0.9.6b5 jailbreak tool.
Update iPhone 3GS, 3G Baseband to 06.15.00 with Redsn0w 0.9.6b5
Step 1
Download iOS 4.2.1
Download Redsn0w 0.9.6b5 for Windows and Mac
Download iTunes 10.1
Note: Windows 7 and Vista users should run Redsn0w as Administrator in XP Compatiblity Mode
Step 2
Restore your iPhone 3GS, 3G to iOS 4.2.1 firmware through iTunes
Step 3
Launch Redsn0w 0.9.6b5 and point it to iOS 4.2.1 to continue the jailbreak process
Step 4
Is this newer (fixed) model? screen will display with a message so read that carefully to select YES or NO. At the moment it can only perform an untethered jailbreak on the older model
Step 5
Click on Next button to proceed for Preparing jailbreak data process
Step 6
Next Redsn0w 0.9.6b5 screen will show different packages options to install. So, check mark the boxes followed by Install Cydia and Install iPad baseband and click Next button and Yes on next screen to confirm the iPhone baseband update to iPad baseband to unlock the iDdevice
Step 7
Please make this sure your iDevice is OFF and plugged in to the computer. Follow the instructions display on screen to put your iDevice into DFU mode and click Next button to proceed
Press Power button for 10 seconds
Without releasing Power button press Home button for 5 seconds
Release the Power button by keep holding Home button for further 13 seconds or until Redsn0w detects your iDevice
Step 8
Now, it will download iPad baseband and your iDevice will reboot
Step 9
Uploading ramdisk process will start and on completion done! message will display so click Finish
Step 10
Now, your iDevice will reboot and then you will find Cydia icon on Springboard
Note: iPhone 3GS New Bootrom users who rely on tethered jailbreak, should select the option Just boot tethered right now from Redsn0w options screen.