A ROM of Windows 10 Mobile build 10240 appears to have leaked onto the internet, compatible with the Lumia 640, 640 XL and 930. According to Gabe Aul, this build has a show stopping bug which is why the build never officially made it out publicly, so I’d avoid installing this if I were you.
Furthermore, since it leaked as a ROM I believe it will require some work getting it installed, as it isn’t as simple as executing the download on your phone. Regardless, the ROM has leaked and is now making the rounds on public file sharing sites. The leak originates from China, for those wondering.
Microsoft says a new official mobile build of Windows 10 will be out in the next few days, the build should be newer than build 10240 as well, which is very interesting. As per all leaks, we will not be providing direct download links.
Thanks for the tip, FaikeeFurther reading: Microsoft, Windows 10 Mobile