How can I change an attribute (ie shirt size) on an order? You wish to change attributes (ie shirt sizes or dinner choices) on orders after a registration has been completed.
To edit a participant's product selection, please go to People -> Participants. There, click on 'edit' next to the participant in question. On the edit screen, you will see a button at the top of the order that reads "Included Items/Questions" - click on it. From this screen, you can update the order as needed.