This 8-inch slate arrived on the market in June 2015, powered by a quad-core Intel Atom SoC and running Android 5.0 Lollipop. Although not available via OTA, an official Android Marshmallow update has been finally issued by Asus and can be downloaded from its website.
http://www.notebookcheck.net/Asus-ZenPad-S-8-0-receives-Android-Marshmallow.174475.0.html