First, move the cursor so that it is inside the window. Now you can pan around by holding down the mouse button and dragging in the direction you want to move. to change your panorama viewpoint. You can usually move up to shift the view towards the sky, down to shift the view towards the ground, and left or right to scan around in a full circle. To zoom in, on the PC: use the Shift key; use Ctrl to Zoom Out. On the Mac: Option key zooms in, Control key zooms out.
All FullScreen panoramas are opened in new browser windows. So the easiest way to get back where you where before is to close the FullScreen window, either using the "Close Window" link, or your PC's close window icon (in the corner of the window)
Not all scenes have enough resolution to appear sharp when shown in full-screen mode. Try zooming out to make the scene crisper.