How do I edit the Roles I am in?

If Role selection was a part of the Signup process you can go back the the Opportunity Public Page and Edit your signup. Doing this will allow you to uncheck the Roles you no longer wish to be a part of. You will always want to un-schedule yourself from the Shifts containing that Role. 

If the Role selection step is not enabled, you will need to contact the Opportunity Manager directly to have them remove you from the Role and any associated Shifts.