and use drop down menu (right) to choose product. You will be then provided with a long (and granted occasionally confusing) list of releases. Compare this with your installed version to see if there is something newer.
However you do not immediately obtain and update...you carefully read the associated release notes to see what might be affected in your particular circumstances as sometimes newer firmware does not support older EOL APs. This can be a serious gotcha in a live production system. Plus the ZD updating and then rollout to APs can take a considerable time depending on how many devices you have. Plannning for downtime of WLAN is also an important consideration here.
There is no automatic update mechanism and the above paragraph should be enough to convince you why automatic updates would be a very, very bad idea to implement.