Warning: Make sure to download exact firmware for your model. You can discern your model from Settings > About phone > MIUI version (Eg: In MIUI version V13.0.6.0 SKHMIXM ‘MI’ represents your phone region, which is Global). You can also verify your Xiaomi phone at official site.
Tip: Some devices might have buggy USB support while in bootloader mode, if you see fastboot hanging with no output when using commands such as fastboot getvar ..., fastboot flash ... you may have to try a different USB port (preferably a USB Type-A 2.0 one) or use a USB hub.
  1. Extract firmware files from zip 
    unzip -qq miui_*.zip firmware-update/*
cd firmware-update

  2. Power off the device, and boot it into bootloader mode

  3. Run the following commands to flash firmware: 
    fastboot flash abl_ab abl.elf
fastboot flash aop_ab aop.img
fastboot flash bluetooth bluetooth.img
fastboot flash cmnlib_ab cmnlib.img
fastboot flash cmnlib64_ab cmnlib64.img
fastboot flash devcfg_ab devcfg.img
fastboot flash dsp dsp.img
fastboot flash hyp_ab hyp.img
fastboot flash keymaster_ab keymaster.img
fastboot flash logo logo.img
fastboot flash modem modem.img
fastboot flash qupfw_ab qupfw.img
fastboot flash storsec_ab storsec.img
fastboot flash tz_ab tz.img
fastboot flash xbl_ab xbl.img
fastboot flash xbl_config_ab xbl_config.img
  4. Reboot: 
    fastboot reboot