1- Charge your device upto 70 percent.
2- Switch off device.
3- Press & Hold Power + Volume down until menu appears.
4- Android Logo with Red exclamation mark appears.
5- Now, Press Volume UP key for a while, to enter Recover mode.
6- Using Volume key select Wipe Data/Factory reset and Power key to confirm
7- Using Volume keys select Yes-delete all user data and Power key for OK.
8- Wiping Data starts.
9- Reboot system now using Power key.
10- Formatting has been done and phone is free from pattern lock.