Your list of articles has a column to the left of the titles called "status". Each article has either a green check or a red X.  

The green check indicates that the article is published. To un-publish, click the green check mark next to the article that you would like un-published. It will turn into a red X, indicating that the article has been un-published.

The red X indicates that the article is un-published. To publish, click the red X next to the article that you would like to publish. It will turn into a green check mark, indicating that the article has been published.