When you join the greenroom for the first time you will be asked to select a camera, but you can change that at any time before or during the broadcast.
Changing your camera
Click the icon under the Effi logo.
This will show you your video feed. Hover for additional options.
Click the quick actions menu and then choose "Video & audio."
This will open up all your video and audio options. You will see what you currently have enabled and be able to select a new camera by clicking the dropdown.
When you are satisfied with your options, just click the "X" to leave and get back to your event. Change your camera and any other options as often as you need to!