You can mark the item as dispatched by opening the original pre-paid Simple Delivery label. It will remain unused and the cost will be automatically refunded to the buyer in 30 days (assuming they paid for it).
The option to upload a different tracking number (when a Simple Delivery label has not been used) is no longer available. You can message your buyer with the tracking details and if necessary, you would be able to add the number to an "Item not received" case if one was opened.