You can clear a tracking column or an entire tracking group of its tracking data by deleting it and then re-adding it. In this example we will be deleting the tracking information under the tracking group called "My Tracking Group", tracking column "1st Course".

1.  You can delete entire tracking groups or individual tracking columns in Team Manager > Settings tab > Tracking sub-tab > [1] Tracking Groups and [2] By Tracking Groups respectively
2. You can also delete tracking data for a course under the Courses tab >  click Menu for a course > Tracking > toggle "Permanently delete this tracker and all associated data" > click Save Tracking Information. 
3. Re-create any deleted tracking groups in the Settings > Tracking sub-tab. 
4. Re-create tracking columns per course under that course's Settings > Tracking tab.