The last year’s flagship, Galaxy S3 and Galaxy Note 2 will get the Android 4.3 Jelly Bean update by the end of this year.
For those of you who can’t wait for the official update to land, the managed to get the test firmware for Samsung Galaxy S4, which is available for download and can be installed following the detailed instructions. The leaked version is for the Snapdragon variant with model number GT-I9505.
The pre-release firmware contains appears to be faster and lag free when compared with Android 4.2.2. Samsung KNOX and Samsung Wallet are also present in the firmware, but you cannot download Samsung from the servers
It is recommended to wait for the official update if you don’t have any experience in following the procedure mentioned at the source link. Based on the information that is popping up all over the internet, it looks like the update will be rolled out very soon.