First method:

  1. Power on the phone.
    Hard Reset SONY ERICSSON C902
  2. Next from the home screen, tap: Menu -> Settings -> General -> Master Reset -> Reset All -> Continue -> Continue again -> OK.
  3. Now your phone will reboots.

Second method:

  1. Back up your important files, and download PC Companion from here: http://www.sonymobile.com/gb/tools/pc-companion/
  2. After installation you will see window with 4 options.
    HardReset SONY ERICSSON C902
  3. Now hover your mouse over the option "Support Zone Now!" and click "Start".
    Factory Reset SONY ERICSSON C902
  4. Next plese wait, until your program Install a suitable additive.
  5. Afterwards from the window with 3 options choose "Phone / Tablet Software Update", and click "Start".
    Master Reset SONY ERICSSON C902
  6. Then click to "repair my phone / tablet".
    Permanently delete data from SONY ERICSSON C902
  7. Accept all informations.
    Wipe data SONY ERICSSON C902
    Format SONY ERICSSON C902
  8. Now your program will run "preparing computer for software update".
    Restore SONY ERICSSON C902
  9. Select your model.
    Remove screen password SONY ERICSSON C902
  10. After that you will see window with informations.
    Remove Screen Lock SONY ERICSSON C902
  11. Follow the instructions, and connect your device into your Personal Computer.
  12. Success!

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

Help! This doesn't work