update HTC One (M7) to Lollipop

Hey All,

I have an HTC One M7 which is unlocked. It's an AT&T phone running on T-Mobile. Current OS is Android 4.1.2. I am unable to upgrade it to Lollipop 5.1.1. I got the ROM from HTC but it won't install. Does any body have any thoughts how to upgrade it to Lollipop 5.1.1

Thanks...