Assignments can be deleted only if you delete all the scores entered in first. From the gradebook, make sure there are no scores for any students in the assignment you wish to delete. Go to your homepage.
At the far right side of your list of classes is a little gear symbol:
Clicking on it brings up the menu where you select Assignment List.
The only assignment that can be deleted (because there are no student scores in it) will have a red X in the Delete column. Click the red X to delete the assignment.