Hello, Mi Fans!
Lightening Fast - MIUI 9, has been started rolling out under Global Beta ROM, know more about it here. While everyone is in await to have the newest iteration on their device, here’s the guide you can follow:

Important points before Flashing,
  • Make sure your battery is above 60%.
  • Always take backup of your Apps & Multimedia content to be on safe side, check the full guide here.

Depending on the ROM version, follow the appropriate method
  • If you are on Developer ROM, Follow 'How to Flash Recovery Method' (Download package with .zip extension)
  • If you are on Stable ROM - Follow 'How to Flash Fastboot Method' (Download package with .tgz extension)

How to Flash Recovery ROM
Method 1
This method applies, when you have the update waiting
  • You will get the notification reading ‘New Update Available’
  • Go to Tools folder - Open Updater app - check for updates
  • If yes, then click download - update.

Method 2
This method applies, when you have the OTA package (Incremental or Full) downloaded via link beforehand.
  • Open updater app - three dots - choose updated package
  • Browse through the files and select the package ending with .zip extension.
  • After verifying the package and your permissions, it will ask you to reboot the device.
  • Reboot the device and your device will boot up to MIUI 9.

How to Flash Fastboot ROM
  • Unlocked Bootloader
  • Download & install Mi Flash Tool - here
  • Download the Fastboot ROM

If you face phone not getting detected,
  • Install Mi PC Suite - here
  • Download Drivers - here

How to Unlock the bootloader
  • Check the full guide here

How to Take Local Backup
  • Go to settings - additional settings - backup & reset
  • Click ‘Local backup - Backup’, to create the backup and move it to PC before flashing
  • Do remember, it includes only apps so multimedia files need to be backup separately
  • Check the full guide, here

Steps to Follow
  • Before fastbooting you must have the bootloader unlocked.
  • Switch off your phone, press and hold volume down and power button simultaneously, it should power up the phone with fastboot screen
  • Connect your phone with USB to PC, click ‘refresh’ button in flash tool app and it should show device ID.
  • If not, install Mi PC Suite and drivers posted above.
  • Extract the fastboot ROM you downloaded, to a folder on PC

  • Open MiFlash tool, and select 'clean all' in the right down corner
  • Click ‘Select’ and browse to a folder where you extracted the Fastboot ROM and click ‘Flash’

  • It will start flashing the phone and will take around 3-5 Minutes depending on the PC and device.
  • Device will reboot with MIUI 9.

Don’t remove the device while flashing is going on, it might take 3-5 minutes.

Credits to R0user