Microsoft has finally released a preview version of Windows 10, with the build number 10549, 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 build 10536.
Windows Insider head Gabriel Aul offered Windows 10 Mobile build 10536 users a way to get the new 10549 build
- Go back to Windows 8.1 first using the Windows Device Recovery Tool.
- Once you're on Windows Phone 8.1, install the Windows Insider app, choose the Fast ring, and reboot your phone.
- After that – this build should download and install on your phone.
"If you choose to remain on Build 10536 – that's fine too. You will not receive today's build but we're working to ensure you'll be able to receive the next one. We know having to go back to Windows Phone 8.1 to get this new build is a bit of work. By doing so you are also helping us test the upgrade path that millions of Windows Phone 8.1 customers will take to upgrade to Windows 10 Mobile, so we appreciate the adventurous Insiders who choose to go this route and give us feedback."