If you want to update your iPhone 4 to iOS 4.1 your baseband will likely be updated from 1.59.00 to 02.10.04 losing carrier unlock.Here is a complete step by step guideline on how to upgrade your iPhone 4 to 4.1 without upgrading its baseband using TinyUmbrella.
The steps are given below.
Step 2: Right Click the TinyUmbrella exe (umbrella-4.1.6.exe) and select “Run as administrator” as shown in screenshot.
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 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.
That’s all.Your iPhone will now be on 4.1 with the 1.59.00 baseband!
You could always get support from UnlockAlliPhone.com about any Unlock & JailBreak issue of any model of iPhone including iPhone 2G,3G & 3GS and also upcoming iPhone 4. They provide Premium Members Support for life time. Means just pay one time a small fee and you will receive support(real human support – avg response time 2 hour) for all present & future version of iPhone.