It’s not a major update but includes the December 2018 Security patch with minor bug fixes and improvements. The smartphones receiving the update includes Xperia XZ3, XZ2, XZ2 Compact, and XZ2 Premium.
It’s an OTA update and released in a phase-wise manner, so it might take some time to reach all users. Once the update arrives you’ll get a notification in the status bar. Click on the notification and it’ll start downloading.
However, you can also check manually by going to Settings –> About Phone –> Software Update.