If you have already connected and joined the meeting then to realize the wrong mic, speak or camera was selected. Here is how you'll be able to adjust while in the meeting.

As the host, click on the gear icon at the top and you'll be brought to your Audio/Video Setup. Make your adjustments then Save.

For Participants, click on the gear icon towards the upper left and you'll be brought to your Audio / Video Settings. Make your adjustments then Confirm.