Xiaomi recently started an alpha and beta test for MIUI 9 with a number of their smartphones. Just yesterday, XDA Senior Member finished gathering up a list of download links for the first batch of devices. This includes the Mi Mix, Mi Note 2, Mi 5, Mi 5s, Mi 5s Plus, Mi 5c, Mi Max 2, Mi Max, Mi 4s, Mi 4c, Mi Note Pro and the Redmi 4x. These download links are for the Chinese fastboot ROM of MIUI 9 beta for these devices and can be flashed with Mi Flash or fastboot if your bootloader is unlocked.
Find these download links in our Android forum