The official ZenTalk forum does not reveal the full changelog of this update, except ZenUI 5.0 and Android 8.1 Oreo. However, it might also include several optimizations, general bug fixes and latest security patches.

Asus starts rolling out the OTA update with build number WW_15.0200.1807.406 to ZenFone 3 Max. Since the OTA roll out in phases, it will take few days to get the update on all devices. Check the update manually by going to Settings > System > System updates. You can also download the firmware (ZC553KL_ WW_15.0200.1807.406) and install manually on your phone. ASUS has made available full firmware for download, which is useful if you cannot install the OTA. Manual Installation Guide: Installing software update manually on ZenFone 3 Max is not a complicated process. All you need to download the correct firmware file for your device’s SKU version and flash it manually. Suggest to backup data before starting the software update.

  1. Download latest firmware update zip file for ZC553KL from Asus website here.
  2. After downloading the update zip file, follow this installation guide. After completing the installation, your ZenFone 3 Max will reboot into the Android 8.1 Oreo based ZenUI 5.0. (Source)