Once you are logged in to your account click on the "Features" tab on the side bar.

Then click on "Polygons".

Under the "Update" column click on the edit symbol of the polygon you want to edit.

​Scroll down to "Map"

Make edits to the polygon by clicking on one of the lines (the polygon will become blue) and dragging it. 

Click on the "save" button on the bottom left and then close the editing screen.

The polygon has now been edited and saved onto the map.