The latest official operating system update available for the Nexus 7 (2013) is Android 5.1.1 Lollipop, which was released in 2015. However, since the device is no longer officially supported by Google, there are no more updates or security patches being released for it. If you want to continue using your Nexus 7 (2013), it may be worth considering installing a custom ROM or alternative operating system such as LineageOS, which may offer more up-to-date features or improvements in performance and security.