If you used the postage label supplied via ebay you don't need to upload. It happens automatically.
If you bought your own label or it was supplied privately you'll need to contact ebay to ask them to change the tracking number on your return. For that you can use callback on here: https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
I have sorted it out now. Thank you very much. All I did was ask the automated assistant to get me a call back and then managed to sort it out with them on the telephone.