The first U. S carrier to roll out the update for Samsung Galaxy S4 is Verizon and the OTA update must be live now. We can now confirm that the leaked document indeed had the right dates published in it where Verizon Galaxy S4 Android 4.3 update was scheduled to roll out on October 28. Other carriers should also expect the update soon.
Since the update is now available, you should receive an OTA update notification like the one you can see in the screenshot below:
If you don’t receive a notification like this, you need to go to Settings>About phone and check for software updates.
Please note that if you had rooted your phone already and installed a custom ROM, this OTA update might not be available to you, In such case, you can flash this OTA update via recovery mode. More information on this can be found here.
It is important to know that if your device is rooted, this OTA update will revoke root access and you might have to root it again but be aware of the KNOX issue before you do that. With this update, your device will be compatible with Samsung Galaxy Gear smart-watch.