To edit a polygon, please follow this actions:

Enable the polygon tool

Click the Edit button on the top bar, or click the 'E' hotkey. The button turns dark green when selected.

1. Double-click a polygon’s contour and it will turn grey with enlarged points: 
2. To shift location: Click and drag any point. 
3. To add a new point :click a line where a point needs to be added. 
4. To delete a point: click a single right-click. 

Once you are done editing a polygon, click Esc and its contour will appear smooth again. 

For more information please go to Polygon