Let's say you accidently set attendance and you want to undo this. You can by rescheduling the event.
You can clear/undo attendance through all 6 calendar views. This tutorial will show you how to do so through your Calendar's Month view.
Click on Calendar > Month view.
Find the event you wish to no longer mark as attended and click on the name of the event. For this example we will click on Frederic Chopin.
Notice that there is a green check mark next to the name of the event. This lets you know that this lesson has been marked as attended.
In the Attended box select the Reschedule radio button.
Click on Save Changes button.
Attendance is now cleared. Notice that the green check mark is gone.