If you used ebay's Simple Delivery the tracking is automatically added. If you didn't use SD but bought your own postage the option to upload a different tracking number is no longer available. You can message your buyer with the tracking details. If necessary you would be able to add the new number to an "Item not received" case if one was opened. Payment may be held for 14 days.
You can mark dispatched by opening the original pre-paid Simple Delivery label (Get prepaid label > Get prepaid label and QR code). It will remain unused and the cost will be automatically refunded to the buyer in 30 days (assuming they paid for it).