If you didn't buy this direct from Yodel, but through eBay then, find your label with this link . You may need to wait 24 hours for it to show up.
https://www.ebay.co.uk/ship/lmng/labels-history/seller-hub and to the left of the sales record there is a drop down menu under 'Actions' click on that , then click on 'Void Label'.
The refund will take between 5 - 7 days to process.
You can then purchase another .