To edit a polygon's shape you first click on the boundary of the polygon you want to edit. You will see white circles appear on the corners of your polygon. You can then click on and drag any of these white circles to where you want. You'll also notice that there are semi-transparent circles midway between all the solid white circles. You can activate these semi-transparent circles by clicking on them once. This allows you to add more shape to your polygon boundary if needed. To delete a white circle click on it once.
See video below for a visual example.