To add the new tracking number you still need to generate the original prepaid label. Then go to the More Actions dropdown menu or View Order Details > Postage > Track Package > dropdown arrow and change the tracking. That needs to be done on a desktop browser.
If the buyer paid for the unused postage label it will be refunded to them automatically in 30 days.
If you don't change the tracking you will have to wait 14 days for the money.