Go to the “Settings” control panel (upper right-hand corner of each page) then look to the left-hand sidebar menu (scroll down a bit) and click on “My Classifieds”. From there you can close, delete, edit, or renew expired ads.
Mike: Only moderators can edit or delete an ad from the ad itself, those options are not available to regular users due to the way the plugin was coded by its author, for our regular users the only place to manage existing ads is through the Settings control panel page.