First method:

1. If the display is off, hold down the Power button to turn it back on.
2. After that, from the Main Menu enter Settings.
3. Then find and select Backup&Reset and tap Factory data reset.
4. In the next step choose Reset device.
5. Tap Erase Everything if you are sure you want to clear all your data. 
6. Finally select OK to confirm the whole operation.
7. Well done! The hard reset has just been performed.

Second method:

1. Start by downloading PC Companion, for example from here: http://www.sonymobile.com/gb/tools/pc-companion/
2. Afterwards install it and run.
3. Next, you will see window with 4 options.
   
4. In the next step 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!