As reported by GetDroidTips, the update is rolling out in India as well as in some other regions and besides the security patch, it also brings stability improvements and system optimizations.
The Mi A1 is Xiaomi’s first Android One based smartphone which promises faster software updates. It was launched in September 2017 and comes with Snapdragon 625 chipset paired with 4GB RAM and 64 GB storage.
There is a dual camera setup at the rear with 12-megapixel sensors, while also houses a 5-megapixel front camera.
The September 2018 security patch for Xiaomi Mi A1 is released over-the-air (OTA) and released slowly in phases so you might get it in the coming days. Your phone will automatically catch the OTA and you will get the notification to install it.
You can manually check update by going to the Settings > About phone > Software updates > Tap on Check for Updates.