Sometimes you just need the added flexibility that an external camera gives you. You may want to shoot a stop animation video with the Stop Motion Animator app, or shoot an action scene with WeVideo, or scan a QR Code taped to a wall. In some of these cases the application will have an option to switch to a different camera built into the UI, but in most situations that is just not the case. Below are step-by-step directions for switching between cameras using Chrome's system settings.
Click on the three horizontal lines (referred to as the "hotdog" by Chrome developers) in the upper right corner of your chrome browser, then click Settings.
Scroll to the bottom of the setting menu and click the Show advanced settings... link.
Under the Privacy section in the advanced settings, lick the Content Settings... button.
Scroll down to Media section and select the external camera from the dropdown menu beside the Camera heading.
Done. Don't worry, this change is not permanent, your Chromebook will automatically switch back to the default camera when the external camera is unplugged.