A few days ago, Xiaomi released an Android 11 updates to its selected smartphones and unexpectedly the Note 9 series is one of the first to get this update. Even Mi 10 series is waiting for it and this is new for the company because in the past they have always rolled out major software updates from premium to budget chronology.
The update itself does not bring a lot of improvements from MIUI 12 apart from the fact that it is based on Android 11. The update comes with the build number V220.127.116.11.RJWINXM and download size is 2.3GB and can be flashed via recovery, however you should wait for the update to appear via OTA.
You can download the Redmi Note 9S/Pro’s Android 11 based MIUI 12 update as a full package from the link given above and flash it on your devices with Twrp. If you have trouble installing ROMs, you can get help from the Xiaomiui Telegram group.