Once you click either the Mic or Cam buttons:
You can click either buttons:
Click "Switch" will bring up the mic and camera test screen, that also allows you to choose other mics or cameras:
You can also click the camera icon in the address bar:
From there you can click allow if you haven't before and you can choose which camera of microphone you'd like to use.
For Firefox it will appear on the left side of the address bar like this:
For Google Chrome it will appear on the right side of the address bar like this: