Remove trim, start points and end points

Window Essentials Movie Maker.  I have set trim or start end points in video. I would like to remove those and begin again.  I have been trying to slide these (white things with stripes) but nothing happens. When I click on a frame and try to slide it only wants to Move the frame.  I think the solution is to slide to the beginning of the source(not sure where source is) to recover trimmed video. My main question I think is, How and what do I slide?  Thank you for your help.