Android 4.3 update is available for Samsung Galaxy S4 via OTA (in batches) and Kies, which you can use to download it if you can’t wait for the OTA to hit your device.
To update your Galaxy S4 to Android 4.3 Jelly Bean, connect your device to your computer and use Samsung KIES or simply use Samsung’s OTA service.
How to download:
To download and install the firmware via OTA, go to Settings > More > About Device > Software update > Update.
All new Android 4.3 Jelly Bean Firmwares for the Galaxy S4 (GT-I9500) are present here, as well. You can use our it to manually update your device if for some reason you can’t officially update your device via Samsung Kies or via OTA as you might be using a Custom ROM or if your country has not yet received the Android 4.3 Jelly Bean Update.