Check this out to download the global beta 6.1.21 http://forum.xda-developers.com/xiaomi-mi-3/general/mi3-4-marshmallow-6-0-5-12-24-alpha-t3280037
Without W that mean your phone is mi4 LTE version, with W mean 3G version.
If you want back to 4.4 follow the instruction that i mentioned on the...