When you click on the "VNC Viewer" tab, you'll see a Window like this:

In this window, you can tell EchoVNC what VNC Viewer you want to use when
you make connections to your EchoVNC partners. By default, EchoVNC will
choose the VNC Viewer that it comes with. But if you're partial to other
flavors or VNC Viewers, feel free to choose a different one.