Unable to upgrade my Android OS

[AC Question]

Hi,

I purchased one AT &T HTC One mobile (OS - 4.4.2) a couple of years back and later I unlocked it from outside. When I try to upgrade my OS using AT & T Software upgrade option, I get " No updates available message".
Kindly let me know how can I force my mobile to upgrade its OS.

 

[jeffhaskins]

I think that means that AT&T didn't release an update for it. To answer your question though, unlock the bootloader and flash a custom recovery with Fastboot. (If you haven't already.) Then make a backup of the stock ROM with your custom recovery. Then download a custom OS such as CyanogenMod 13 (Marshmallow) or any of the other ROMs available for you device and install through custom recovery.