When you first arrive in the studio, you should have a local feed in the top left corner (see pic)
See the two inverted triangles? Those are pull down menus that allow you to change the video and audio source.
Note: you CANNOT change your video or audio source while your feed is in use in either the Scene Builder, Live Monitor, or persistent audio (the Audio-Only Bar). To change your video or audio source during the broadcast, you'll need to switch to a different live feed, pre-recorded video, or image, and then make the change.