Hi
If you want to downgrade because your current ROM isn't satisfied enough..
Then check out this guide:

Official Flashing Guide to Flash Xiaomi Mi/Redmi Phones Using Fastboot Method
http://en.miui.com/thread-960590-1-1.html
[Source: Xiaomi MIUI Official Forum]


But first, you need to unlock your bootloader..
Also, save your data too..