For those of you who upgraded to iOS 6.1.6 but forgot to save the SHSH file for it, you’ll not be able to untethered jailbreak the device and unlock your iPhone. Therefore, you can only be running on 4.1 or 6.1.6 only. Currently, there is no tools to create an iOS 6.1.6 Custom Firmware. So if you want to update to 6.1.6 then just restore the original firmware using iTunes.
Unlock iPhone 3GS running on iOS 6.1.6
Step 1: Download Rebsnow 0.9.15b3 and unzip it
Step 2: Once the software is launched, click on the “Extras” button
Step 3: Download iOS 6.1.6 from here and click Select IPSW > Choose the firmware > Click Yes (if your device is 3gs oldboot, then click No).
Step 4: Now go back to the main menu, select “Jailbreak” > give a check on install cydia and Install iPad baseband (the purpose is to get Baseband 6.15) > Next.
Note: When selecting Install iPad baseband, Redsn0w will ask you a Yes or No question, just select Yes.
Step 5: Turn off the device, put it into DFU mode and wait for the jailbreak process to be completed. If you still not see the Cydia icon on your Springboard, make sure to do a quick “Just boot”.
Step 5: Now go to Cydia and let it reload data and upgrade everything, then go ahead and install a utility called ultrasn0w.
Step 6: Still on the same Redsn0w, choose Extras > Select IPSW > click on the origianal iOS 6.0 firmware > Click No
Step 7: Repeat step 4 and 5, then you’re done. You have successfully downgrade from Baseband 6.15 to 5.13. Enjoy!