Open the Dashboard via Live Camera, the status menu or in a document’s sidebar.

Click the pink bar along the top of the microphone button.

In the panel that appears, click Disconnect.

You can now select a different microphone.

Why is it like this? In other tools it can be easy to end up on the wrong microphone without noticing, which is the easiest way to quickly degrade the quality of a presentation. CueCam makes this a two step process so you can be confident that the correct mic is being used.