To edit an active Ad or Draft, go to the Manage Ads section of your account. If you are unsure on how to get to this section please see this article.
From the manage ads page, find the Ad you would like to edit. Your most recent ads will be shown at the top of the page with older ads below them. Find the Ad you would like to edit and select the EDIT button. If you cannot find the Ad you are looking for, you can search for it using the provided search bar at the top of the page. A new view like this should appear:
Select EDIT UPGRADES in the UPGRADES section. It will bring you back to the original options page and you can select any new options or change current upgrades. When finished, select next at the bottom of the page to go to the PREVIEW page. If you are satisfied with the changes, select NEXT again to go to PAYMENT. Use default Payment or enter new information, and when finished select PAY AND POST AD.