[dalastone]

1. turn off the power
2. press and hold the "volume down" key and "enter key", then press the "power button"
3. release the power button but continue to hold the "voume down" and enter key" til you see a message/prmpt saying "this operation will delete all personal data...."
4. release "volume down" and "enter key". then press the "volume up" key to befing the process.
or just go to start/settings/ system/ clear storage" and follow prompts.