Under the Content heading, select Article Manager
Select the existing article that you wish to edit (you may need to go to another page or change the viewable number of articles to find the one you wish to edit)

You can change the Title of the article, change its location in a section and/or category, change if it is published or if its displayed on the front page
After making any changes that are needed to the article, click SAVE in the Top Right Toolbar to save the changes