Your cluster consists of multiple machines ('nodes'). Upgrading is as simple as adding a new, upgraded node, then evicting everything from one of the existing nodes, then take it down. Repeat until every node is replaced.

Downtime is the same as with a deploment, so if you run at least 2 copies of everything there should be no downtime.

As for updating the images of your containers, you build them again with the newer base image, then deploy.