Verizon started updating its version of the handset a few days ago, and AT&T followed suit today, but the updates will roll out gradually, so don't worry if your handset is still running an older OS version.
In case you're wondering, GDR2 comes with a few notable updates compared to its predecessor, such as native support for MKV video files, a new searchable Settings screen, app permission toggles, support for external Bluetooth keyboards, the ability to pin individual settings to the home screen, as well as a feature that protects the device against unauthorized resets.
Back in August 2014 when HTC launched the One M8 for Windows, the company announced that it will continue to update the smartphone for two years following its launch. With Windows 10 Mobile rumored to launch in September, reason dictates that the One M8 for Windows is set to receive an update to the next mobile OS from Microsoft as well.
source: Windows Central