Today, open beta tests of the MIUI 6 have finally started but these are only for Mi 4 and Mi 3 (China Unicom and China Telecom Editions) users.
Starting at 5 PM today, owners of the Mi 4 and the Mi 3 open edition (China Unicom/China Telecom) can upgrade their phones to the MIUI 6. Users of the stable editions will be unable to upgrade to the MIUI 6 by OTA at this time but they can use the 4.8.29 open version to experience the MIUI 6.
Take note. After upgrading, please use default theme. Third party themes may be incompatible. News of an upgrade for the Mi 3 China Mobile Edition may come during the mid-autumn season.