To edit or delete existing content, log in and then bring up the page you wish to edit. Look below the article (or article summary) for a link that says "administer", or sometimes "edit this page". Depending on your user permissions, you might see this below all pages or only certain ones (like those that you yourself submitted).
Clicking this link will bring up a page with a form for changing the page. To edit the page, change the text or settings and then submit. If you wish to delete the page, look for a "delete" button near the bottom of the page. When you click it, you'll get a second chande to confirm that you wish to delete the page--or to change your mind!