>How do I edit the video where I can go only to web cam for a short period and then back to both screenshare and webcam?
Mark, could you clarify what you are asking? It sounds like you have screenshare in the background, with webcam in a smaller window in the foreground. Now you want to show the webcam only for a while? If that's the case either of these will work:
Split the Webcam track at the in and out points. Scale the webcam to fill the screen in between.
Insert a Custom animation at the in and out points on the Webcam track, use that to scale your Webcam.
If I got it wrong please post an image or video with a bit more description.