You must have Admin Level - Webmaster access to un-commit an athlete.
Click Events & Competition > Team Events from the left navigation menu.
Find your desired event and click its Edit Commitment button.
Click on the Committed Athletes tab
Select the athlete you want to un-commit from the event.
From the "Athlete Signups" page, clear all approvals by check-marking the event and clicking the Clear button.
From here, you can un-commit the Athlete by using the Signup Record and selecting No thanks XXXX will Not attend this event.
Click Save Changes.