In Chrome, go to the open meeting and click on the camera icon in your address bar. There, you can go to Manage and set the microphone or camera Demodesk should use. After you made your changes, go back to Demodesk and refresh the page.
In Firefox you are asked right away which microphone or camera you want to use in Demodesk. If you wish to make changes to these settings in a running meeting, simply reconnect to the webcam or audio.