You need to install TWRP Recovery, boot to recovery, install
xiaomi.eu_multi_libra_6.1.21_v7-5.1.zip, wipe cache and dalvik, reboot system.
http://en.miui.com/thread-169401-1-1.html
You could try without wiping user data, if problems occur then have to wipe. Download xiaomi.eu_multi_libra_6.1.21_v7-5.1.zip to phone internal memory first.