You can't downgrade that easily. As far as I know, the only solution would be to unlock your bootloader and install an older custom ROM, but I could not find any for this device during my quick search.
Unlock bootloader, download the ROM you want (from sites like Xiaomi firmware updater or MiFirm) and flash it via fastboot. Done. Now relock your bootloader with miflash_unlock.exe (if you want to).