A family of Microsoft word processing software products for creating web, email, and print documents.
Well, for one thing, if you want a different polygon, you might do better to start with one of the other Basic Shapes (which include pentagon, hexagon, etc.). But I'm guessing you must have hand-drawn this rectangle instead of using the Rectangle shape (which doesn't permit node editing AFAICS). But the context menu you get when you right-click on a line segment gives you the option of Straight Segment or Curved Segment.