Xiaomi's Mi 6 received a beta build of Android 9 Pie back in April, and it looks like the beta testing has concluded successfully, because the stable update is now rolling out. It is reaching global Mi 6 units as MIUI 10.4.1.0, and it's a 1.6GB download. As usual with over-the-air firmware releases, it might take a few days before every single Mi 6 unit out there receives the update notification. If you're impatient, you can hit the Source linked below for a direct download link and accompanying installation instructions. Don't expect to see any visual differences once you have...
Source: https://www.gsmarena.com/xiaomi_mi_6_gets_stable_android_9_pie_update-news-38295.php