Sony is expected to update its Android devices with Android 4.3 this month and to Android 4.4 KitKat at some point in the future. Sony announced earlier that its recently released phones will get Android 4.4, but they have to be updated first to Android 4.3. The official update of Android 4.3 for Xperia Z (C6603) and Xperia ZL (C6503) have now leaked online.
The firmware update files are available through XDA forums, you can download the update if you own one of these devices. Firmware build 10.4.B.0.569 is the Android 4.3 build for both the devices, but it is not clear whether this is the final build or not. It is not clear what are the changes in this version, but expect a lot more changes under the hood. Sony might have introduced some minor UI changes as well.