Read on to know how to download and install Android 9 Pie (MIUI 10.2.2.0) on Xiaomi Mi 8 and Mi 8 Pro. Xiaomi starts rolling out the OTA update in phases, so it will take a few days to get it on your phone. You can either wait for the OTA update or download MIUI 10.2.2.0 for Mi 8/Mi 8 Pro from below link and flash manually.
MIUI 10.2.2.0 update log: Related Post: Redmi 6 and 6A Received MIUI 10.2.2.0 Stable Update (Download and install)
Download Android Pie (V10.2.2.0) for Mi 8/ Mi 8 Pro
Download Xiaomi Mi 8 MIUI 10.2.2.0.PEAMIXM Recovery ROM [HERE] Download Xiaomi Mi 8 Pro MIUI 10.2.1.0. PECMIXM Recovery ROM [HERE] Ensure the following before flashing: – Make sure that the device has at least 60% battery left. – Create a backup of all files and data using Mi back up option
How to Install MIUI 10.2.2.0 (Android Pie) on Mi 8/Mi 8 Pro
- Download the Recovery ROM (.ZIP file) and save it on a folder in Internal Storage of your phone. 
- On your phone, go to Settings > About phone > System Updates or open the Updater app and click on three dots at the top right of your screen. 
- Tap on the option “Choose update package” and then navigate to the folder where you have saved the ZIP file. 
- Select the file, and flashing will start. It will take 5 to 10 minutes to complete the flashing process. NOTE: If you have installed TWRP custom recovery on your Mi 8 or Mi 8 Pro, then you can also try to install ROM zip file via it. Boot the phone in to TWRP, select Install, navigate and flash the ROM zip file.