Power down the phone.
Press Power + Volume Plus button combo for about 5 sec's. You will get to green screen; don't let go of the buttons. After another 5 sec's display will turn red.
Let go of buttons, and a menu will be shown with various options for installing update/ reset/ cache wipe / data wipe etc.
Move to data wipe option using Vol +/- options. And select the option using power key.
Below the options, the result can be seen. Once data wipe is done, select Reboot phone option.
After reboot the phone should return to factory condition.