Can't Update factory unlocked att HTC One m7

[AC Question]

I bought an unlocked ATT HTC One m7 and it has android 4.4.2 loaded onto it. I want to update it to the lollipop update, but it's on MetroPCS so I can't use ATT software update. Tried to use RUU on HTC website but I keep getting ERROR 155:. What do I do? Thanks!

 

[Rukbat]

Is the phone rooted? If so, you can't update it. (Updating a rooted phone could brick it. You'd have to flash a stock ROM - back up everything first, because you'll lose it - then update.)