It is very important that you correctly define and assign who will be giving the webinar.
If you have already assigned it and you want to change it, you can do so at any time before the room starts, since once it is started it will no longer allow you to do so. The rooms start 20 minutes before the start time you have set.
To modify it, simply go to the "actions" button and select "modify" and change it in the corresponding section. Make sure to save the changes.
NOTE 1: To add a second speaker in the webinar room, see how to do it here