You can cancel your volunteer task by following these simple steps.

1. Click on the Class name in the left navigation tray
2. Click on Signups in the Updates tab
3. Click on the Volunteer Event you signed up to be a volunteer of
4. The Volunteer Event details screen is seen
5. Scroll down to see the Items/tasks you signed up for
6. To cancel your sign-up, tap on the task/item you signed up for
7. Tap Can't Bring This Item or Can’t Volunteer and the Sign Up is canceled
8. The creator of the Volunteer Event will be notified about the cancellation. 

For more information, check out the following articles:

Requesting Volunteers for Recurring Tasks in Class
Requesting Volunteers for My Class Event
Canceling My Volunteer Sign Up
Asking a Question on a Volunteer Task
Assigning Parents to Volunteer Tasks
Editing a Volunteer Event