The International Samsung Galaxy S III, GT-I9300 is receiving its first leaked build of Android 4.3 Jelly Bean. The build carries the I9300XXUGMJ9 software version and is said to be stable although there could be a few bugs inside it.Before attempting to install this update, users are advised to perform a full backup of their devices, as this firmware will wipe your device. While it is not an official release from Samsung, installing it will not void your warranty. If you can't wait for the official release you can download the update using the following link and follow the source link for further information
Once you install the update you will find is that the UI has been changed to the one used on the Samsung Galaxy S4 and the Samsung Galaxy Note 3 with a new tabbed interface for the Settings application.
The launcher and widgets remain the same as the ones used previously on the Galaxy S III, and secure business application Samsung KNOX has yet to be included in the test builds of Android 4.3, although it is expected in the final build.
source - SamMobile | AndroidAuthority