How to install Windows 10 on any Lumia

The First Technical Preview of Windows 10 for phones was released last Thursday, but it was limited to only a select few Lumia handsets. Fortunately the folks at XDA-Developers have managed to find a workaround, allowing Windows 10 for phones to be installed on just about any Lumia.

[NOTE] Keep in mind that Microsoft does not support this method, and you could end up making your smartphone unusable.

[NOTE] Installing Windows 10 can also void your warranty, so please note that you are proceeding at your own risk. This website, nor the creators of the mod will be responsible for any loss of data or device malfunction.

With the warnings out of the way, people have actually reported success using this method on various phones, including the Lumia 720, which features only 512MB of RAM. Other device owners who have successfully installed Windows 10 include those with Lumia 930 and Lumia 1320 handsets, available through various carriers. However some are reporting issues on the Lumia 920 and the Lumia 520.

[NOTE] The following guide has been tried and tested on the Nokia Lumia 1520 on AT&T. While the method should technically work on any Lumia smartphone (Windows Phone 8.0 or higher), it may require some tweaking. Please note that you are proceeding at your own risk.

[NOTE] Installing Windows 10 for an unsupported device will mean that certain functions (WiFi, Audio) may not perform correctly. Also keep in mind that this is a prerelease version of Windows, so their are likely bugs that need to be fixed before the build is usable.

1. Download and install the Windows Phone Recovery Tool.
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2. Backup all your important files and data on your Lumia device.
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3. Reset your Lumia device using this guide. You will have to login to your Microsoft Account, but do not restore your apps or settings just yet. It is recommended that you setup your device as a new device.
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4. Download and install the Windows Insider app from the Windows Phone Store.
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5. Open the Windows Insider app, click on the Options menu, select 'About' and make note of your details. You can perform a screenshot or email your details to yourself.
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6. Download the WPInsiderHacks app from XDA, and extract the files to your computer.
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7. Run the WPInsiderHacks, and approve any firewall requests.
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8. Connect your Lumia to the same WiFi network your PC is connected to. On your phone, edit the settings for the WiFi connection to turn on the Proxy and set it to use the PC’s IP address and enter 8877 for the port.
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9. Open Internet Explorer on your phone and navigate to http://192.168.1.2:8877 where you will replace 192.168.1.2 with the IP address of your PC running the WPInsidersHacks app.
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10. Tap on the link at the bottom to the fiddler root certificate.
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11. When prompted click open and then install to trust the cert and OK at the confirmation.
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12. Run the Windows Insider app and tap get preview builds.
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13. Tap on which custom action you'd like to perform. In my case I hit "Set to ATT Lumia 635" and click the arrow at the bottom.
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14. Accept the agreement and click the check mark at the bottom. the app will close.
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15. Now clear your proxy settings on your Lumia and run the insider app again. This will restore the internet connection on your smartphone and allow you to download the preview builds.
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16. Tap get preview builds and login with your Microsoft account.
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17. Select the branch you want to receive updates through and click the arrow at the bottom.
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18. Now go into the phone settings and check for updates as you regularly would. With any luck you should have Windows 10 there for you to install. Older updates may have to be installed first.
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19. After you have installed Windows 10, it is recommended that you perform another reset and login to your device as you usually would.

If you have any issues with the guide, please follow the source link below and check back with its original poster for updates.

source - XDA Developers