You need to generate the original prepaid label to be able to change the tracking number and mark as dispatched.
So generate the label then go to View Order Details > Postage > Track Package > dropdown arrow and change the tracking. That has to be done on a desktop browser.
Afterwards if you paid for the original label you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
Or if the buyer paid for it it will be refunded to them automatically in 30 days.