Students
If you are a student, the easiest way is to go to your course page and find the "My Articles" section.
To remove the old one, click the plus/minus button, and then click the minus button next to the title of the article you want to remove.
To add the new one, add the title of the new assignment you want to add to the input field and click Assign.

Instructors
Go to the "Students" tab, then click "Assign Articles" - this will open up a pop up view within the Students tab.

From the new view, you can click the plus/minus button next to each student to update the article they've been assigned or peer reviewed. You can remove an article using the (-) button or add a new article by entering the article title in the box and clicking "assign" - when you're done, be sure to click DONE in order to save your work!

Good luck!
-Wiki Ed Team