There are four ways Xiaomi update:
If you want to install a Xiaomi update using recovery – download the official update, reboot your phone or tablet into recovery mode and click “Install update from SD card”.
With the system update tool you can flash full Xiaomi firmware, download it here:
and save it into internal storage, go to the settings – about phone – system update – choose update package – select downloaded rom, press “OK” and wait for the end of updating.
If you want to flash stock firmware for Xiaomi Redmi using Fastboot – download and unpack ADB&Fastboot, install Drivers. How to flash stock firmware Xiaomi Redmi using Fastboot instruction is here (you need to unlock the Xiaomi bootloader).