Click the “+” icon, then “Screen Share”, the other participants will be seeing your screen instead of the webcam feed.
Once you have clicked “Screen Share” the browser will ask you to confirm what exactly you would like to share:
(example from Chrome)
Please make sure to choose an appropriate screen and click “Share”. If you wish to stop screen sharing, please click again on the “+”icon and choose either “Camera” for webcam feed or “Off” for audio-only option.