Switching between main video clip and smaller inset smaller clip

Hi I am trying to create a video using two video clips. One occupying full screen and the other which is smaller and inset into one corner. At several points during the video I would like to switch these videos around so that the larger one becomes the smaller one and vice versa. Does anyone know of an online tutorial out there to achieve that effect, or explain in easy terms to this novice. Thanks in advance