Microsoft has released a new preview version of Windows 10 Mobile, with the build number 10581, to members of the Windows Insider Program who have signed up to get updates in the Fast ring.

While this new build does not have any new features, Windows Insider head Gabriel Aul stated:

"We've fixed the bug preventing upgrades from earlier builds. So if you stayed on Build 10536 or if you flashed back to Windows Phone 8.1 and then installed 10549 or 10572 you can upgrade directly to this new build without going back to 8.1 again."

Source: Microsoft