Update [Oct 24]: Android 8.1 Oreo with ZenUI 5.0 is now available for ZenFone 4 Max ZC520KL model too as version 15.2016.1809.407.
The update is available over the air and installs software version 15.2016.1809.412, but we still don’t know when the ZC520KL model will join the party, although we expect it to. Like its counterparts, the update also installs a new ZenUI 5.0 skin on the ZenFone 4 Max alongside plenty of bug fixes and security enhancements.
Related: ZenFone 4 Android Pie update news
Being an OTA update of a major OS, expect a huge file. To avoid any interruptions, make sure you have connected to a stable Wi-Fi network to upgrade to Oreo 8.1. You can check if the update is available for your device via Settings > System > System updates.