Windows 10 Mobile is being made available to supported smartphones running Windows Phone 8.1.
Microsoft listed eligible phones and they are all phones with at least 1GB of RAM

Phones and devices that can upgrade to Windows 10

Lumia 1520, 930, 640, 640XL, 730, 735, 830, 532, 535, 540, 635 1GB, 636 1GB, 638 1GB, 430, 435, BLU Win HD w510u, BLU Win HD LTE x150q, and MCJ Madosma Q501

Have one of the phones mentioned above? Get the Windows 10 Upgrade Advisor app- available free in the Windows Store- to easily check for upgrade availability and prepare your phone to install the upgrade.

    If you cant find the Upgrade Advisor on Windows Store and our device is among those listed as eligible for upgrade then first install “find Upgrade Advisor App” so you can easily locate the Upgrade Advisor app easily on Windows Store

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