According to AT&T, this update will bring the following changes to the Galaxy S4:
- Android 4.4 OS upgrade
- Samsung Galaxy Gear compatibility
- Drag and drop to uninstall or disable applications
- Call reliability improvements
Similarly, you can also manually check for the update by going to Settings > About device > Software update > Check for updates. Do share with us about the update once you’d installed it on your device!
Source : AT&T