How to Upgrade to MIUI 9
1. Please don’t forget to back up your data before upgrading to MIUI 9.
2. If you are using MIUI V5 / MIUI 6 / MIUI 7 / MIUI 8 stable ROM, please upgrade to MIUI 9 via fastboot method.
3. If you’re using MIUI 8 Global Beta ROM, please go to ‘Updater’ and check for updates.
4. If your device is locked, please unlock it before fastboot flashing. How to unlock your phone bootloader? Click here to check the details.
5. How to flash MIUI ROM? Click here to see detailed MIUI flashing tutorials or check this thread to see the step-by-step guide!

MIUI 9 Global Beta ROM 7.8.10 Full Changelog
MIUI 9 Lightning fast
New - Deep-level system optimizations (08-07)
New - New icons (08-07)
New - New animations for launching and exiting apps (08-07)
New - Simpler Home screen editing (08-07)
New - Introducing Split screen (08-07)
New - Improved Silent mode triggered by the volume button (08-07)
New - Improved readability for the start page of Messaging (08-07)

System
Optimization - Devices with rare Bluetooth profiles are hidden in the device list (07-25)

Settings
New - Search for apps in the Dual apps settings (08-03)

Note: The above info is about MIUI Global Beta ROM, please click here to check the latest update for MIUI China Developer ROM.

MIUI 9 Global Beta ROM 7.8.10 Download Links
Xiaomi Mi6
Recovery 1460M Download
Fastboot 2035M Download

Redmi Note 4X
Recovery 1424M Download
Fastboot 2020M Download

Get it on Google Play
https://play.google.com/store/apps/details?id=com.miui.enbbs