A ROM for Windows 10 Mobile build 10240 has leaked and is compatible with the Lumia 640, Lumia 640 XL and Lumia 930. However, it's recommended you stay away from these latest leaks as quite the bug has been detailed by Gabe Aul in this particular build, hence why it hasn't hit official channels.
Originating from China, this leaked ROM is making the rounds, but you needn't bother. As noted already, we recommend you disregard these ROMs (and like WinBeta we shall not link to them) simply because issues are known and they could mess up your smartphone.
That, and a new version of Windows 10 Mobile will hit official channels in due course. We'll be sure to give you all a heads-up when the next build is officially out. Stay safe out there.