If you want to change your camera in Google Meet while you're not on a call, click More > Settings. On the left, click Video > Camera, select the Detail Camera and hit "Close". 

To change your camera while you're not on a call, head to meet.google.com, click Settings > Video. Here you can select Detail Camera. If your camera is working properly, you should see your video feed on the right. Click Done to confirm your camera selection.