The Xiaomi Mi A2 is supposed to receive the Android Pie update before the end of the year, but the firmware still needs some work. For now, the official Beta ROM can be downloaded via TWRP or fastboot and tried by the more adventurous users. Obviously, there is no OTA update to talk about at this stage.
https://www.notebookcheck.net/Android-Pie-Beta-for-the-Xiaomi-Mi-A2-rolls-out.361291.0.html