To edit a polygon first select it.
A selected polygon
Drag either of the polygon corner manipulators( 
) to change the corners or ends.
Drag any rotate manipulator 
to rotate the polygon.
Drag any scale manipulator
to re-size the polygon.
Drag the polygon to move it.
Polygon Properties Dialog
You can also edit the parameters of the polygon using its properties panel as shown below. To view the properties panel first select it.Then right-click on it, then select Properties Panel from the context menu that opens.
Polyline/Polygon Properties Dialog
Click the 
button to show this help topic.
Vertex
This is the selected vertex/segment. The selected vertex is drawn as a solid red square. The selected segment id drawn in red.
X
The X coordinate of the selected vertex.
Y
The Y coordinate of the selected vertex.
Click 
to add a vertex.
Click 
to delete the selected vertex.
Check/uncheck 
close/open the polygon.
Click 
to fillet the polygon
A filleted polygon