When you first arrive in the studio, you should have a local feed in the top left corner. The video and audio sources that are used for that local feed are your default camera and mic.
But what if you would like to use a different camera or microphone? You can do this by adding a new local feed.
Click the 'ADD OR INVITE FEEDS' button
Click the 'ADD LOCAL CAM' button
Use the drop-down menus to select the camera and microphone
You can close the local feed that you don't need by clicking the small 'x' in the top left of the video card.