Finally, Zenfone 2 owners can also enjoy Android 6.0 Marshmallow. Asus has started to roll out the Android 6.0 Marshmallow OTA (over the air) update for ZenFone 2 ZE551ML 4GB RAM variant. Though, currently the update available for Android Marshmallow Beta testers only, other owners of ZenFone 2 need not to wait too long to get the update.
The update weighs about 1.2 GB and upgrade Zenfone 2 ZE551ML to Android 6.0.1 Marshmallow with June security patches. Android Marshmallow brings various new features (new personalized permission system, doze battery life saving, now on tap, etc.), changes and performance boost to the device.
Note that the update will replace some ZenUI apps, such as Asus Messenger, Asus Mail and Asus Calendar to Google Messenger, Calendar, and Gmail. Also, many apps will no longer be available once you upgraded your ZenFone 2 to Android 6.0 Marshmallow.
Also, note that the internal storage will decrease slightly after the update, so backup your files before updating. The Marshmallow update will not support APP2SD, so move apps to internal storage before updating.
Update ZenFone 2 ZE551ML to Android 6.0 Marshmallow
1. The update requires at least 2GB free space on the internal storage, so free up enough space before upgrading your Zenfone 2 to Android 6.0 Marshmallow.
2. Now, if you have not received the FOTA notification on your device, check it by going to Settings > About > System updates.
3. If the update found, let it download and install.
If you have any problem updating your ZenFone 2 ZE551ML to Android 6.0 Marshmallow via FOTA, you can update manually by downloading the firmware [HERE]. However, note that the Android Marshmallow update not yet live at Asus servers, we will update it when it will become available (source – 1, 2)