Step One: Download Xiaomi stock fimware for your device here
Step Two: If you downloaded the firmware to your PC, copy it to the root of your SDcard. If you downloaded to your device, skip to next step.
Step Three: From your launcher Homescreen, click Tools, then select Updater
Step Four: Now tap Menu button on your device and click Select installation file
Step Five: Navigate to the folder / directory where the downloaded firmware is located
Step Six: Select the firmware package and tap Update button
Step Seven: Now wait patiently for the update process to complete.
Step Eight: Once it's complete, tap Reboot to new version.
If you have any questions or comments, please post in comments section below! I value your comments.