Open the Article you wish to delete

From the Article Options menu (right-most green button on the top of the page), select "Edit"

While editing the Article, click the red Delete button (trashcan icon) on the top right corner of the page. 

Confirm that you would like to delete the Article by clicking the checkbox next to "Yes, I am sure".

Choose whether or not you would like to Purge the document, which makes the deletion irreversible, but also free's up the Article's slug for use by a new Article.