Owners of the Samsung Galaxy S4 on AT&T will be pleased to hear that they are the first to get an update to their device. Users who have already upgraded their software are reporting that the update brings with it faster performance and smoother animations.
The 21MB doesn't come with a changelog, so it's hard to definitively say what Samsung has updated in the latest version. The update should be available over-the-air, though users can also update it using Samsung's desktop Kies software. To download the OTA update, head to Settings > More > About Device > Software update