We're working to get more Windows Phone models supported worldwide. Download the Windows Insider app, and when your phone is supported you'll be able to sign in with the same Microsoft account you used to join the Windows Insider Program. Windows 10 Mobile Insider Preview currently works with the following phones—as long as they have at least 8 GB of storage, Windows Phone 8.1 was already installed on the phone at the time of purchase, and the version number is 8.10.14219.341 or earlier:
HTC One (M8) for Windows
Lumia 640 XL
Lumia 950 XL
Some phones, regions, and mobile operators might not support Insider Preview. If your phone isn't listed, and no preview builds appear under Enroll in the Windows Insider app, then your phone isn't supported and it may or may not be supported in the future.
To see which Windows Phone operating system (OS) version is installed on your phone, go to Settings > About > More info.