Go to your Purchase History, find the return and select "View return details". Under the tracking info select courier from the dropdown list and enter the new tracking number. Then mark as sent.
If you have any problems with that contact eBay on your phone only (not a laptop or PC) to ring them or request they ring you. So view this link on your phone:
https://www.ebay.co.uk/help/eua?id=5190
They should be able to change it for you.
@angusmacqueen