1) Turn off the device
2) Now hold down the buttons volume up + power button for a few seconds (do not press the power button for a long time)
3) Keep pressing the volume button up until you meet the "EMUI" image next to the Recovery mode.
4) Go to the Wipe Data / Factory Reset option and confirm with the Power button.
5) The procedure will take one or two minutes
6) Now click on Reboot System Now
That is all. Remember that this procedure will delete all data from the device.