Under the Control Panel:

1. Click on Course Tools to expand this section.
2. Select Achievements from the list of options.

You will now see a list of Achievements within the course. To delete an Achievement, hover over the Achievement you wish to delete and click the chevron button to the right of the Achievement.  From the menu that appears, select Delete.

You will then see a dialog box similar to the one above asking you to confirm the deletion.  Click the OK button to confirm.