Preserving your iPhone 4′s baseband is one the best things you can do for you and your iPhone. Read the following on how to update your iPhone 4 to iOS 5 safely.

There are 5 different baseband versions since the iPhone 4 was released more than a year ago. They are 1.59.00, 2.10.04, 3.10.01, 4.10.01 and the latest being 4.11.08. All baseband versions can be unlocked except the newest that comes with iOS 5. That’s why it’s important to preserve your baseband before updating to iOS 5.

Follow the step-by-step instructions below:

1. Download and install iTunes 10.5 from Apple’s website.

2. Be sure to backup all your data, contacts, photos etc.

3. Download custom iOS 5 firmware for iPhone 4. This is a ‘cooked’ firmware with the baseband update portion removed. There are 2 different types that you can choose:

• If you have the official carrier SIM card, download sn0wbreeze_iPhone 4_nojb-5.0.ipsw
• If you don’t have the official carrier SIM card, download sn0wbreeze_iPhone 4-5.0.ipsw this is a “hacktivated” and “jailbroken” firmware.

4. Download the latest redsn0w from the Dev Team’s website.

5. Start redsn0w, go to Extras > Pwned DFU. This allows your iPhone to accept custom firmware. If you don’t do this, you will get error 1604, 1600 etc.

6. Connect your iPhone to your computer.

7. Hold shift (or Option on Mac) and click Restore. Select the custom iOS 5 IPSW you just downloaded.

8. Once the restore is complete, you can then activate it with your official carrier SIM card.

9. If you installed sn0wbreeze_iPhone 4-5.0.ipsw, it will be activated automatically. Please note that the jailbreak is “tethered” which means you need your computer to reboot your phone to a jailbroken state. To do this, launch Redsn0w, go to Extras and Just Boot. Follow on screen instructions.

You will notice that your baseband version has not changed. This allows you to unlock your iPhone.