To draw the premise, Click on the drawing icon on the Top-Right of the map.
Drawing the premise will require at least 5 points (last point closes the polygon).
To modify the Premise polygon, you can click and drag the points that you created.
To delete the entire polygon, Use the Trash Icon (click the Trash Icon and then click the polygon) to delete. Additionally, you can also click it and hit backspace.
To finish (or update) the setup of the site, click to save changes.