Official Changelog:
- Manage phone settings more easily through the categorized and searchable settings menu.
- Bluetooth keyboard support added (Bluetooth HID 1.1 profile)
- Support for MKV video playback
- Change the phone name in Settings > About.
Lumia 635 and HTC One for Windows owners on AT&T started receiving the GDR2 update some days ago. Earlier Microsoft had announced GDR2 roll out to Lumia 735 and Lumia 830. Software update trackers have also been updated with GDR2 build 8.10.15148.160 roll-out info for these two Lumia devices.
You may like to read all the details about Windows Phone 8.1 update 2 aka GDR2 at our dedicated page.