Found out today when I decided to check for updates. Settings > Scroll all the way down to About phone > Click on System updates at the bottom.
What's new
MIUI 9 Lightning fast
New - Deep-level system optimizations(08-07)
New - New icons (08-07)
New - New animations for launching andexiting apps (08-07)
New - Simpler Home screen editing (08-07)
New - Introducing Split screen (08-07)
New - Improved Silent mode triggered by thevolume button (08-07)
New - Improved readability for the startpage of Messaging (08-07)
Highlights
New - Reading mode can turn on and off atsunset and sunrise automatically (05-03)
New - Improved layout and readability forsearch results in Contacts (06-29)
New - Virus scans and payment scans weremerged into Security scan (06-29)
New - Better blocklist rules with thepossibility of separated rules for every SIM card (07-10)
New - Switch for turning Battery saverautomatically when charged (07-19)
New - New display formats for update logsin Updater (07-25)
New - Search for apps in the Dual appssettings (08-03)
New - Mi Picks gets a new name and a newicon! Meet Apps! (08-21)
New - Quick ball goes back to the edgeafter 3 seconds of inactivity (08-30)
Optimization - Cleaner removes emptyfolders better now (07-10)
Optimization - Protection fromunintentional tapping for Dial pad (08-08)
Optimization - Redesigned home page (08-24)
Optimization - Devices with rare Bluetoothprofiles are hidden in the device list (08-29)
Optimization - Simplified and improved thedesign of the virus scan result page (09-07)
System
Optimization - Increased system decryptionspeed (06-23)
Optimization - Devices with rare Bluetoothprofiles are hidden in the device list (07-25)
Optimization - Devices with rare Bluetoothprofiles are hidden in the device list (08-29)
Fix - Home screen loaded too slow withAndroid 7.1 (06-23)
Fix - Apps changed Wi-Fi connection statuswithout the user's permission (06-23)
Fix - Developer options couldn't be openedusing Bahasa Indonesia (08-15)
Fix - Music stayed in the Notificationshade after exiting the app (08-15)
Fix - Only Mi TV/Mi Box can be added/shownon Mi Remote app after the update. (08-22)
Fix - WhatsApp force closed in Split screen(08-29)
Fix - Fingerprint sensor works faster now(09-07)
App Lock
New - Added the App lock step beforeunlocking using a new fingerprint for the first time (08-15)
Contacts
New - Improved layout and readability forsearch results in Contacts (06-29)
Optimization - Protection fromunintentional tapping for Dial pad (08-08)
Fix - Contact search didn't work properlyfor Russian and Ukranian (08-29)
Messaging
Fix - Time in Messaging didn't change whenthe system was switched to 12-hour format (08-29)
Fix - Messages with one-time passwordsdisappeared (09-07)
Lockscreen, Status Bar, Notification Bar
Optimization - Notification shade response(07-04)
Fix - Female voice for English (Waiting forWi-Fi) in the Notification shade (11-02)
Fix - Music player stopped working on theLock screen after 10-15 minutes (11-02)
Home screen
New - Animated icons for selected apps inthe default theme (09-01)
Optimization - Folders open faster (07-10)
Optimization - Users can set their mainHome screen in the editing mode (08-29)
Optimization - New animation for launchingand closing apps (09-01)
Fix - Couldn't close Recents in some cases(05-09)
Fix - Issues with icon size for Clock,Calendar, and Weather (05-16)
Themes
Optimization - Redesigned home page (08-24)
Camera
Fix - Brightness jumps to maximum whenviewing photos (09-12)
Fix - Language support issues (09-12)
Optimization - Improved design for Feedback(05-24)
Settings
New - Reading mode can turn on and off atsunset and sunrise automatically (05-03)
New - Search for apps in the Dual appssettings (08-03)
Optimization - Introducing Button andgesture shortcuts (09-15)
Optimization - Media volume can be turnedback on using volume buttons in DND mode. (09-14)
Optimization - Floating notifications arerestricted by default in DND mode. (09-14)
Optimization - Vibration response in DND mode.(09-14)
Optimization - Improved timers forSilent/DND. (09-14)
Fix - Issues with ringtones in dualWhatsApp (05-24)
Fix - Android security patch level wasshown with the wrong date (09-07)
Screen Recorder
Fix - The recording couldn't be playedregularly after the process of recording was stopped by locking the device(05-12)
Optimization - Launching speed (07-01)
Downloads
New - Downloads keep waiting when theconnection is established again in case it was interrupted (09-05)
Browser
Fix - Issues with Urdu (05-24)
Recorder
Fix - Adjustments for Recorder and Secondspace (06-23)
Calendar
New - New cards: Horoscope, News, Cricketnews, and Cricket matches (05-18)
New - Holiday cards for Russia (06-28)
New - Introducing football cards with theinfo about major international leagues and matches (08-15)
Blocklist
New - Better blocklist rules with thepossibility of separated rules for every SIM card (07-14)
Cleaner
Optimization - Cleaner removes emptyfolders better now (07-10)
Battery Usage
New - Switch for turning Battery saverautomatically when charged (07-19)
Security
New - Virus scans and payment scans weremerged into Security scan (06-29)
Optimization - Simplified and improved thedesign of the virus scan result page (09-07)
Scanner
Fix - Scanner couldn't work normally(09-12)
Quick Ball
New - Quick ball goes back to the edgeafter 3 seconds of inactivity (08-30)
Other
New - Mi Picks gets a new name and a newicon! Meet Apps! (08-21)
Fix - Google widget disappeared because ofthe Dual apps (06-20)
If for some odd reason you can't receive the update via the updater app, you can do the following to get the MIUI 9 Stable ROM:
You do not need to unlock your bootloader or have root access to do this. Always remember to backup your data and files before attempting this just to be on the safe side.
Edit: You may need to be on the latest update prior to this new one for this to work