Click the "Article Manager" button.
This will show you all the articles you have access to.
Find the article
Use the search filter or any of the filter dropdowns horizontally across the top of the article manager. The articles can also be sorted by any of the column headings in the table (Title, Ordering, Date for example)
Click the article title to edit it. Save changes.
Editing features are the same as creating an article.
Unpublished: click the green checkmark. It will turn to a red target. Or unpublish multiple articles at once by clicking the checkbox to the left of the article titles. Click the appropriate action button in the upper right (Unpublish or Trash).