That info is out of date.   eBay have removed the option to change tracking number.    All you can do is  message your buyer with the tracking details.   However,  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.

List with free postage and buy your own postage outside of eBay.   To get the buyer's address and mark dispatched you can open the original prepaid label (Get prepaid label > Get prepaid label > Print label).  

You can claim a refund for an unused (unscanned) label here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller