So first of all I want to inform you that all the data or contacts of your mobile phone will be lost after hard resetting your mobile phone.
1) first of all turn off your phone and make sure your mobile phone is fully turned off.
then press and hold together volume up key and power key for a few seconds.
2) Now release both the buttons when you see mi logo appears on your Mobile Screen.
3) wait for some time then you will see some options like wipe data and reboot system on the screen.
4)Now select wipe data option by using volume up and volume down key and then press the power key to Ok.
after that select all wipe data option and press power key to ok.
5)Now you will see two options confirm and cancel. Select second option confirm and press the power key to OK.
6) Now you will see the option like back to main menu select that option and press the power key to OK.
5)Now select reboot and press power key to OK after that select reboot system and press the power key to OK.