This is the official Kitkat Firmware from Sony for your Xperia Z .
New Features
1. Improved performance & user interface (UI) optimization.
2. Tweaked, improved and current experiences for native apps like Messaging, Email, Smart Social Camera, Battery STAMINA Mode, Sony’s Media apps: WALKMAN, Album and Movies, with Sony Entertainment Network cloud service integration, Sony Entertainment Network & PlayMemories integration with a more intuitive UI.
3. Better integration with Sony PlayStation 4.
Compatibility with SmartWear Experience
Before you start make a backup of your data because everything will be deleted from your internal storage .
Also make sure that all the necessary drivers are installed for your device on your computer .
Steps
2. Download the official Kitkat ROM ( 10.5.A.0.230 ) from here
3. After downloading the ftf file for your device copy it to the flashtool location is C:/Flashtool/Firmwares.
4. Now run Flashtool.exe and connect your device and make sure flashtool detects your phone . This ensures that all the drivers are properly installed . If your phone is not detected then go to C:/Flashtool/Drivers and install necessary flashmode drivers .
5. Now click on lightening button on top of flashtool and select flashmode .
6. Now select the firmware which you downloaded and leave options as default.
7. Now a pop up will appear prompting you to connect your device.
8. Connect your device while holding the volume down key.
9. Flashing will start and as it ends disconnect your device and restart it.
10. You will have the latest Kitkat firmware installed in your device.