AT&T HTC One M8 owners should be receiving a sizable update today. The update touts Android 4.4.3 as well as general bugfixes and under the hood tweaks.
The update clocks in at 685.5MB, so make sure you are connected to WiFi and have your M8 wither fully charged or plugged in to a power source. Once the update is ready for download, you’ll receive a notification and can begin installation. If you’re not into waiting for things, you can always head to Settings on your device and check and see if there’s anything you can manually download there.