Reach out to the delivery partner directly to coordinate a return of the order to the restaurant. You can reach out to them by going to the "Order history" section of the restaurant dashboard (the clock icon in the lower left of the screen). "Order history" includes a list of all past orders and will have the phone number of delivery partners currently en route to your customers.
If you are unable to contact the delivery partner directly, call Uber support. We will contact the customer directly and let them know to expect a delay due to the mix up.