First, go to your "Assignments" page, where you can edit, update, or delete your assignments.
If the assignment is still in-progress, hover your mouse over the assignment you want to delete and click on the pencil icon in the bottom right corner. A small screen will appear and you will then see a "Delete Assignment" button. Click on the button and the assignment will be deleted.
If the assignment has been completed, go to your completed texts under "Assignment Status" and you will see a "Delete Assignment" option next to each text. Click on that option and the assignment will be deleted.