I must warn that guide below is a little technical, do NOT proceed if you have no confidence doing so. I bear no responsibility if you brick your phone and render unusable. Please also consider the following before you proceed:
- You will lose your root if rooted - no worries, here is the video guide to re-root.
- You will lose all data in internal storage - do a full backup.
- You will have to reinstall all apps just like the phone is freshly out of box - do a full backup.
- The entire process takes 2 hours.
- Fully charge your phone before upgrade.
- You may need a new phone if you fail to upgrade.
- This is a Windows guide, I do not know how it works on non-Windows platforms.
The entire upgrade process comprises of 2 phases:
- Change the AP firmware to British Retail version so that the OTA upgrade file for Europe can be used to upgrade the phone to JB 4.1.2.
- Upgrade to JB 4.1.2.
8. Launch RSD Lite. Click ... button.
10. Click Decompress only.
13. Press volume - button to highlight AP Fastboot. Press volume + button to select/enter.
17. Now press all 3 buttons simultaneously like in #12 above to enter into Boot Mode Selection Menu.
18. Use volume - button to highlight Recovery. Press volume + button to select.enter.
19. Press both volume + and volume - buttons simultaneously when you see the Andoird bot with exclamation mark. This will bring up the Recovery Menu Options.
20. Use volume - button to scroll to "apply update from external storage". Use Power button to select.
21. Select by pressing Power button the OTA upgrade file you've downloaded in #7. The upgrade takes quite a while, about 20 minutes. You will see screen below during the upgrade...
22. Once complete, use the same method to "wipe cache partition" and "wipe data/factory reset".
23. Select "reboot system now".
Congrats! You are now on Jelly Bean!! Enjoy!