If you've already signed up for an Event, or for an Organization, but need to make changes to your form for some reason or another, you can do so by going back to the Organization, or Event signup page and clicking 'Edit Signup'. This will allow you to go through your entire registration form and edit any information within it. If the Event/Organization Manager has allowed volunteers to select their own Shifts, you can also come back and sign up for other Shifts here.
Of course, you can only edit these previously answered questions. So, if you need to make any other changes that may be out of your control or you need any clarification, you can always contact support or the Event Manager(s). The contact information for your Event Manager can be found if you scroll down slightly on the Event Page or you can use the integrated 'Contact Manager' button.