1. First of all switch off the device by pressing the Power button.
2. hold down Volume Up + Power button for about 10 seconds.
3. You can release held keys when you see the Android Robot.
4. If you see a menu, navigate with UP/DOWN keys and go for a factory reset.
2nd Method.
1. Enter the Recovery Mode hold down the Power key for a short while.
2. From the menu choose "Wipe Data/Factory Reset" by using Volume buttons to scroll and Power button to select options.
3. Tap "yes" to confirm the operation if you are certain you want to clear all your data.
Use the Power button to accept option "reboot system now".