Microsoft is now rolling out the Windows 10 Mobile preview build 14291 to nearly all of the devices that are able to receive updates from the Windows Insider program



From Windows Central:

The one exception is the Lumia 635, which will get the build at a later date. Users will need to be on the Fast ring to see the update. Build 14291 is part of the new Redstone branch, which is set to unify OneCore and bring new native features to Windows 10. Previously, only a subset of new Lumias that launched with Windows 10 Mobile were eligible for the preview update.

  Windows 10 Mobile preview build 14291 released to more Fast ring Insider devices