You can access the stream settings by clicking on the cog at the top right of the video widget, where you should be able to change the selected video source. There’s a picture of this in the docs 
There are also some joystick button functions being added to switch between video sources, but I believe those are only in the development builds at the moment - they may not yet be available in a stable version.
Note that changing video source also stops the current video recording - it can be easy to get caught out by that if you’re not aware of it.
Note also that it’s possible to use other programs to receive additional streams, which may be worth looking into if you want to see (and/or record) multiple at once