Volunteers can select their shifts at the time of application, if you have the option enabled. If you would like them to edit shifts after they have been approved, you need to make sure that in the application that volunteers are allowed to edit their application after approval. Once that option is enabled, volunteers can go to https://www.eventeny.com/users/application/ and select "View or edit" on the application for your event. They can then click "edit" on the shift selection section and select their shifts from there.
Hope this solves that for you! If you have any further questions though, please reach out to firstname.lastname@example.org. We're here to help. :)