This guide is based on this guide on 4pda

Important Notice

Instructions

2. Download the MiFlash tool
3. Download the device's official firmware in .tgz format
4. Unpack MiFlash and create a log folder in it's directory
5. Open XiaoMiFlash.exe
6. When promted to install drivers, do that
7. Unpack the firmware (you may have to do that twice)
8. Press "select" and select the firmware dir
9. Choose the flashing mode: "clean all" to wipe user data, "save user data" to keep it
10. Do not choose the default: "clean all and lock". It will lock the bootloader

11. Plug in your phone while in fastboot mode
12. Press "refresh" and "flash"
13. error: flash timeout , error: Not catch checkpoint (and probably others) are ok. They just mean that the device wasn't lock