Microsoft is very close to release Windows 10 Mobile update for existing Lumia running on Windows 10 Mobile. Along with Lumia, there are many devices which are currently running on Windows Phone 8.1 update, including HTC and Blu devices.
@StefanMalter There are no further updates planned for the HTC Windows phone 8X currently ^CD
— HTC UK (@HTC_UK) January 11, 2016
Recently, HTC’s official Twitter handler confirmed that Windows 10 Mobile update is not hitting HTC 8X devices. However, HTC covers a very small amount of Windows Phone market share and user of HTC 8X are negligible. It is worth to note there is no confirmation regarding when other OEMs like Blu will release Windows 10 Mobile update for their devices.