See the below article from Christian Post
The Galaxy S4 and Galaxy S3 Android 4.3 update schedule for AT&T, Verizon, T-Mobile and Sprint leaked onto the internet this week.
The Galaxy S4 should start receiving the update on 10/28 on Verizon, 11/13 on AT&T, 10/30 on Sprint and 11/18 on T-Mobile. The Galaxy S3 will get it on Verizon starting on 11/22, AT&T on 11/13, Sprint on 11/6 and T-Mobile on 11/25.
Both of these devices will have the update before the end of December. The update is already available on other versions of the handset as Android 4.3, JellyBean started rolling out for the LTE international model last week.
The first S4 to receive it is the unlocked version with the model number GT-I9505. The firmware is only available in Germany at this time, but it should start rolling out in other markets during the coming weeks.
The software brings many changes to the software including KNOX, Galaxy Gear compatibility, improved performance with graphics, improved stability of user interface, and security.
Once this update is installed, users will not be able to downgrade, according to the change log for the software