At the moment the absolute latest version of Android is 5.1.1, which is already out for some devices. However, the HTC-made Nexus 9 is now in the process of receiving a software update to Android 5.0.2.
Throughout its short existence, the Nexus 9 previously only got one of those, bumping its OS version from 5.0 to 5.0.1. The update to Android 5.0.2 Lollipop is on its way to Nexus 9 owners, and it’s around 23MB in size. As the minor version number jump from 5.0.1 to 5.0.2 implies, this one should be all about the bug fixes.