Microsoft has finally released a preview version of Windows 10 Mobile, with the build number 10572, to members of the Windows Insider Program who have signed up to get updates in the Fast ring. However, before you download, be aware this is a single update from Windows Phone 8.1, and cannot be used to upgrade from previous older builds.
To move up to the new build, you'll have to roll back to Windows Phone 8.1 using the Windows Device Recovery Tool. From there, you can install the Windows Insider app, select Fast ring, and the build should be available for download after you reboot your phone. If you happen to need a little assistance with that process, be sure to check out our guide for more.
Source: Gabriel Aul (Twitter)