update at&t one m7 from android 4.1 to 4.4 topic

hi guys
i have already read many tutorials but i just want a push to get started.
here is my problem my friend bought an unlocked at&t htc one m7 from ebay and using it in india. once it reached india it haven't received any ota updates after that so its still stuck on 4.1.
i just want to know how can i update it to 4.4 directly by flashing a stock firmware. in samsung its just like dwnload and flash using odin,in nexus its just fastboot. i am a bit confused here with ruu,cid,s off etc. i just want to update it to a international/asian version so it can get future otas.
any help regarding this is appreciated.
thanks