Xiaomi has officially started the release of MIUI 9 Global ROM around the world. It is currently available for the Redmi Note 4 and Mi Max 2 users. If you own any of these two smartphones, you should look for the OTA update notification or you can simply check for it manually by going to Settings -> About Device.
As for other devices, Xiaomi has said that MIUI 9 will be released for the Mi MIX 2, Redmi Y1, Redmi Y1 Lite, Redmi 4, Mi 5, and Mi Max starting mid-November. And for all remaining eligible devices, it will be made available by early December. It is worth mentioning that the update will be released in batches to users. It means even if the update is rolling out for the device you have, it might take some time for it to arrive on your particular unit.
MIUI 9 Global stable ROM was made official yesterday during the Redmi Y1 launch event. It was previously in beta testing since its unveiling in July this year. The latest MIUI ROM is focused on offering faster speed and smoother performance but also comes with a number of new features and a lot of improvements over predecessors. It includes support for stock Android-like notification management, 21 new animated icons, instant switching between the two split-screened apps, and improved inbuilt photo editor among other changes.