HE Instructor Knowledge Base URL: http://cmg.screenstepslive.com/s/MacmillanMedia/m/52444/l/609708
Click the section title
From your Courses page, click the title of the section where you want to unassign a lab. This will take you to the Assignments page.
Click the gear icon and select Unassign
The lab will be removed from the Assignments list and again appear as unassigned in the lab picker drawer.