Before you continue reading any further please understand that anything you do to your smartphone is your own responsibility as the procedure described above can be harmful, void your warranty or even render your smartphone useless. That being said, we understood the disclaimer and got ourselves a Windows Phone 7 Mango-powered LG Optimus 7 (which was unlocked as in not tied to any carrier at all). You can update to Mango too, just read below.
You don’t need to have a developer unlocked smartphone or pre-tampered with in any way, it worked on our stock LG Optimus 7 just fine. While there are several ways to achieve this, the easiest is outlined in the steps above:
Prerequisites: Read the disclaimer if you skipped! Other than that, you just have to have a NoDo-powered Windows Phone and make sure you’ve got plenty of battery juice left!
1. Download the tool found at the source link, version 0.2 of the tool with backup ability, right between “Update” and “Notes” (not hot-linking the file)
2. Install Windows Phone Support Tools, 32bit or 64bit flavor according to your OS
3. Unpack the tool to C:
4. Run the Tool downloaded in step 1 using Administrator rights and let if finish.
5. Once finished it will start Zune
6. Check for updates and apply Windows Phone 7 build 7401 update found
7. Once finished, check for updates again and apply Windows Phone 7 Mango build 7661 update found.
8. After the Update the Zune software itself will try an update and, while it will say it is up to date, it won’t find your phone. You need to manually update Zune to Beta Mango client (build 4.8.2098.0).
Word of note: According to Microsoft, you will have to revert to NoDo before Mango is officially out. Make sure you put the tool you downloaded in step 1 as well as your device backup from C:PreMangoState in a VERY safe place. Follow the instructions in the readme of the tool to revert to Mango.
Source: Windows Phone Hacker