Hard Reset SONY Xperia Z1 C6903

First method:

1. If the display is off hold down the Power key to turn it back on.
2. Afterwards go to Settings and then to Backup & Reset.
3. In this step tap Factory data reset and Reset Phone. 
4. Then, choose Erase Everything if you are sure you want to clear all your data. 
5. Finally, select OK to confirm the whole operation.
6. Well done! The hard reset has just been performed.

Second method:

1. At the very beginning you should download the PC Companion, in example from here:http://www.sonymobile.com/gb/tools/pc-companion/
2. Then, you should install and run it.
3. Next you will see window with 4 options.
   
4. Choose option "Support Zone New!", and click "Start".
   
5. Wait for the program to install the appropriate add-on.
6. After that you will see next window with 3 options.
   
7. Go to option "Phone / Tablet Software Update", and click "Start".
8. Select "repair my phone / tablet".
   
9. Now follow the instructions and accept all informations. 
   
   
10. Please be patient and wait until preparing computer for software update is complete.
    
11. Afterwards choose your phone / tablet model.
    
12. If you see this window, then connect your device into your PC.
    
13. Good job!

Third method:

1. Firstly, download flashtool from Sony website:http://developer.sonymobile.com/services/flash-tool/
2. Make sure, that you have installed drivers for your USB data cable.
3. Afterwards turn your device off.
   
4. Then press and hold Volume Down, and connect to USB cable.
   
5. Follow the instructions on your screen, and accept appropriate options.
6. Success!

WARNING !
Hard Reset will erase all of your data
All described operations you are doing at your own risk.