You cannot change the image once submitted. You will have to upload the new design and create a new shirt listing. If you need to change some verbage, you can edit the title, features, price, and description. You cannot edit the brand name though.
It depends on what you want to edit... If there is a problem with the actual design, then you will have to delete it and create a new one with the changes. If you want to change the Title, price, key feature, or description, then go to the manage tab, find the shirt you want to edit, then click on Actions, then edit. Make changes, Save, then confirm changes.