1.Backup your pictures and contents because this procedure will delete all of your phone's contents.
2.Unplug usb cable from PC, Turn off your phone, hold volume up and volume down buttons then press down power button. the Fastboot menu will be shown on screen.
3.Connect usb cable with Linux PC. Use Fastboot tool to unlock the bootloader(Fastboot tool already included in your android SDK)
# fastboot devices
List connected device
# fastboot oem unlock
This will unlock the phone's bootloader
4. Use Volume up/down buttons to select at "Yes" option and pressing power button to unlock the bootloader. Now your old contents will gone forever.
5. Restart the phone. Now you have to do fresh set up for your phone.
After unlock bootloader, you can now install other images on the phone.
Have fun !
