All your available inputs are displayed as small previews below the outgoing video. You can tell the the current video input by the red border, and the audio by the speaker icon.
Switch video by clicking the video input you like to play out. You can also use the numeric keys, i.e. press 2 for input two.
Switching video and audio
You can switch both video and audio by pressing CMD + numeric key, i.e. CMD + 2 to switch video and audio to input two.
Select the previous video input by pressing the numeric button 0.