You can update your return address here:
https://accountsettings.ebay.co.uk/uas/addresses
It's not possible to change it retrospectively for the label that's been issued, and your buyer can't send it back without a label, so I'd contact eBay and ask them how to proceed.
I’ve found that the easiest route to contact eBay Customer Services is via the link below where you can request a call back:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid