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The effect Windows 10 has on your Windows-based mobile device -- whether it's a Windows Phone, Surface or Surface RT tablet -- varies based on device type.
With Surface RT tablets, nothing will happen: Microsoft discontinued the Surface RT, so there is not a Windows 10 upgrade for those devices, and they will continue to function the way they always have.
In the months immediately following the release of Windows 10 for desktop computers, we probably won't see anything happen to Windows Phone devices either. Microsoft has said that devices running Windows Phone 8.1 will be eligible for an upgrade to Windows 10 Mobile. Devices running Windows Phone 7 are not eligible for the upgrade to Windows 10. Windows Phone 8 users cannot upgrade to Windows 10 Mobile directly, but they can upgrade to the Windows Phone 8.1 OS and then to Windows 10 Mobile.
Another reason there won't be much change around Windows Phone devices right away is that mobile providers are likely to wait a while before they allow users to install Windows 10 Mobile on their devices. When Microsoft released the Windows Phone 8.1 OS, it took many months before some mobile providers made the update available to customers. Windows 10 Mobile will surely be available to mobile users, but it may take some time.
Users can upgrade their Microsoft Surface and Surface Pro devices to Windows 10 the same way they'd upgrade a Windows 8.1 desktop or laptop. That is, all available updates must be installed to the device, one of which is the Get Windows 10 app. It lets users reserve a copy of Windows 10 and coordinate the upgrade process.
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