First, download the latest stock ROM and extract it. Now, install all necessary USB drivers to connect your device to the computer. Back up all your data. Turn off your phone and go to fastboot mode or Boot into recovery. Now, select the Wipe option to factory reset your phone. After that, you need to select the Install option from the recovery home screen, and navigate to pick the stock ROM you downloaded earlier. Finally, Swipe the bar to start the installation process. Now, you can reboot your phone when it's finished.
You have done it successfully, Enjoy your device with a fresh look.