Go to My Commitments in the browser that you used to sign up for your slot(s):
https://participant.ivolunteer.com
Click on the upcoming volunteer slot and then click the Cancel link at the bottom.
Note:  If you DO NOT see a Cancel link next to a commitment it could be because:
The event does not allow self cancellations
OR, it’s too close to the event to self cancel.
In either of these cases, you should contact your event coordinator if you need to cancel.