As the screenshot above shows, a new firmware is available for the Zenfone Zoom as build WW-18.104.22.168, and it brings Marshmallow update to the device.
Asus has happily axed some serious bloatware too in the update, as it did with the Zenfone Selfie Marshmallow update today.
Before attempting to update, make sure that your Zenfone Zoom is a WW variant. Look at the software version on your device under Settings > About device, it should start with WW. And your device’s model no. should be ZX551ML.
Also, because apps2SD feature will no longer be supported on Marshmallow, you MUST transfer all apps on storage (thanks to apps2SD) to internal storage before installing Marshmallow update.
For changelog, have a look at the pic at the top.
→ Download Zenfone Zoom Android 6.0 Marshmallow update | 22.214.171.124
To update your Zenfone Zoom to Android 6.0 Marshmallow, download the update from above, and follow the guide given at this page.