First, go to your "Assignments" page, where you can edit, update, or delete your assignments.
If the assignment is still in progress, find it under the default In Progress view under the Assignment Status header over on the left. Then, find the assignment and click on the trash can icon in the upper right-hand corner of the assignment. A small screen will appear confirming whether you would like to delete the assignment and any existing student data for the assignment. Click on the "DELETE ASSIGNMENT" button and the assignment will be deleted.
If the assignment has been completed, click on Completed under the Assignment Status header. Then find the assignment and follow the steps above.