If you have the iPhone 4 and you want to update to iOS 4.1, your baseband will likely be updated from 1.59.00 to 02.10.04 losing carrier unlock. Updating to iOS 4.1 using TinyUmbrella will protect your baseband from update and later allowing you to carrier unlock your iPhone 4. Follow the tutorial on how to upgrade your iPhone 4 to 4.1 without upgrading its baseband using TinyUmbrella.
How To Unlock iPhone 4 on iOS 4.1
Step 2: Right Click the TinyUmbrella exe (umbrella-4.1.6.exe) and select “Run as administrator” as shown in screenshot. If you get a Windows Security Alert, make sure both boxes are checked, and hit Allow Access
Step 3: Make sure your iPhone 4 is connected, so that TinyUmbrella can detect the type of device and its firmware.
Step 4: Make sure Cydia or TinyUmbrella has an SHSH of the latest firmware. You can do this by simply clicking “Save my SHSH”.
Step 5: Hit Start TSS Server. Once The TSS Server is “running” successfully, umbrella will kill iTunes. Leave TinyUmbrella Open.
If you do receive an error, DO NOT continue. Your restore will not work if the TSS server cannot be started.
Step 6: Open iTunes and Hit Shift+ Restore. Browse to and select the iPhone 4.1 Firmware File.
Hit ok to Restore iPhone
Step 7: iTunes should give you error 1004. This is what we were looking for. Notably, the baseband has not updated.
Step 8: Go back to the TinyUmbrella window and Click Kick Device Out of Recovery. If you have issues here, try closing and re-opening TinyUmbrella.
Your iPhone will now be on 4.1 with the 1.59.00 baseband! Now, You can follow this guide on how to carrier unlock iPhone 4.