Since Xiaomi officially announced MIUI 9, the company has been slowly rolling out the Global Beta version of the UI to supported devices. The rollout has now reached the final phase, and has now been made available to the third batch of smartphones that are eligible for the MIUI 9 Global ROM 7.9.7 update. These are the last devices that Xiaomi plans to update to Android Nougat.
Xiaomi had revealed its plans to upgrade a bunch of smartphones to Android Nougat earlier this year, but said this will be a gradual release that will happen in three stages. Xiaomi Mi 6 and Redmi Note 4 were the first to receive the update, followed by a second wave of devices such as Mi MIX, Mi Note 2, Mi 5, Mi Max 2, and a couple of others. And now the third wave of Xiaomi devices are eligible for the MIUI 9 Global Beta, and it is also the largest rollout.
In the third wave, the list of smartphones that can now be upgraded to MIUI 9 Global Beta 7.9.7 include, Xiaomi Mi 5, Xiaomi Mi Max (32GB), Xiaomi Mi 2, Xiaomi Mi 2S, Xiaomi Mi 4, Xiaomi Mi Note, Xiaomi Mi 4i, Xiaomi Redmi 3, Xiaomi Redmi 3S, Xiaomi Redmi 3X, Xiaomi Redmi 2, Xiaomi Redmi Note 3 (Snapdragon), Xiaomi Redmi Note 3 Special Edition, Xiaomi Redmi 4G, Xiaomi Redmi Note 2, Xiaomi Mi Max Prime, Xiaomi Redmi Note 4, Xiaomi Redmi Note 4G, Xiaomi Mi Note 2, Xiaomi Mi 5s and Xiaomi Mi 5s Plus. Do note, some of these smartphones are not available in the Indian market. 
Other than the global rollout of the update, Xiaomi also announced a MIUI 9 China Beta version for some low-end and mid-range smartphones. These include the Xiomi Mi Note, Xiaomi Mi 4, Xiaomi Mi 3, Xiaomi Mi 2, Xiaomi 2S, Xiaomi Mi Pad 3, Xiaomi Mi Pad 2, Xiaomi Mi Pad 1, Xiaomi Redmi Pro, Xiaomi Redmi Note 4X MTK, Xiaomi Redmi Note 4, Xiaomi Redmi 4A, Xiaomi Redmi 4 Prime, Xiaomi Redmi 4, Xiaomi Redmi 3S, Xiaomi 3X, Xiaomi Redmi Note 3, Xiaomi Redmi Note 2, Xiaomi Redmi Note 4G, Xiaomi Redmi Note, Xiaomi Redmi 3, Xiaomi Redmi 2A, Xiaomi Redmi 2, Xiaomi Redmi 1S, and Xiaomi Redmi 1.
It is essential to note that this update isn’t of the OTA nature, and you will not exactly be notified to run and install the beta. You will need to flash this ROM if you wish to update to the MIUI 9 Global Beta.