1. turn off machine.
2. press and hold vol + and - button then press and hold power button.
3. at this screen press vol+ to enter "recovery mode".
4. death android will appear on screen then press power button to go to recover list menu.
5. at this screen use vol button to choose "wipe data/factory reset".
6. use vol button to choose "Yes -- delete all user data" then press menu button(soft button on the machine panel) to confrim it.
7. wait a moment, then screen will back to recovery list menu. Choose "wipe cache partition" then press menu button to do it.
8. it take a shortly time, then screen will back to recovery list menu. Choose "reboot system now" then press menu button to restart machine.
9. This process will take serveral time, go to smoke a cig or have a coffee...Finish.