For those people that want to keep their same baseband on their iPhone 4 GSM or iPhone 3GS but want to move on up to iOS 5.1 here is how you do it using Windows.
Step 1: Download RedSn0w
Step 2: Download iOS 5.1 firmware for your iDevice. Place it on the desktop so its easy to find
5.1.0 (3GS): iPhone2,1_5.1_9B176_Restore.ipsw
5.1.0 (4 GSM): iPhone3,1_5.1_9B176_Restore.ipsw
5.1.0 (4 CDMA): iPhone3,3_5.1_9B176_Restore.ipsw
5.1.0 (4S): iPhone4,1_5.1_9B179_Restore.ipsw
Step 3: Extract the RedSn0w archive then right click the RedSn0w executable from that folder and select Run as Administrator to launch
Step 4: Click Extras from RedSn0w
Step 5: Click Custom IPSW
Step 6: Select your iOS 5.1 you downloaded
Step 7: Click OK once it’s done creating
Step 8: Click Pwned DFU
Step 9: Connect your iDevice and power it off. Click Next
Step 10: Follow the instructions on the screen
Step 11: Once it’s “Done!” open iTunes and click the icon for your iDevice
Step 12: Select your device, hold down the Shift key and click Restore
Step 13: Select the NO_BB custom IPSW that was created
Step 14: Let it do its thing, then it’s done, you are updated to 5.1 yet same baseband
Stay tuned with us for more breaking news. You can follow us on Twitter, Facebook, Pinterest, YouTube and check out our RSS for quick reference.