Just after, HTC One M8 users started receiving Android Lollipop update in Malaysia, AT&T also released an updated for M8 users in US, though not Android Lollipop. The upgrade, which collides the Android OS variant to 4.4.4, improves battery operation, enables HD Voice characteristic and introduces Bluetooth & Security improvements.
The other changes the upgrade brings along include Usage Manager Application, Copy and Paste camera feature, and add-on of AMBS Cloud Messaging Integration, also updates to Visual Voicemail and AT&T Ready2Go.
If you owned an HTC One (M8) from AT&T, you can check updates reaching to Settings ->> AT&T Software Update ->> Check for updates.