Official Android Pie Closed Beta Update with FM Radio Support & New Swipe Up Pie Gestures rollout started for Xiaomi Mi A1 users.
The same beta versions is now being rolled out as stable OTA Update for Mi A1 users
Installation Guide : You can only flash this rom with unlocked bootloader
You can get the OTA on your phone but I would suggest you to wait for few days n try anything as downgrade to oreo is messy right now
To get the Pie Update on Your Phone ::
Reset - No sim - Location services off - Connect to WIFI - VPN - Netherlands / USA - Check for Updates