Note:- It's Beta version. After success full testing of beta it will be released for all user as stable version. If you are familiar with beta ROM you can flash your device with the help of above thread.
Currently there is No Official Announcement for MIUI 9 Stable.
However, MIUI 9 Beta ROM is out & can be flashed. MIUI 9 Stable might take some time.
OTA update is released for Stable MIUI versions, MIUI 9 is under testing & has some bugs to be removed before Stable release.
If you're still interested then Flash the latest MIUI 9 Beta ROM available