Update your return address here for future packages.
https://accountsettings.ebay.com/uas/addresses
On this package your options are limited, but I would get in touch with the current resident of your old address and ask them to hold the package for you to pick up, or ask if they can ship it to you if you send them a prepaid shipping label.
Ultimately if you can't retrieve the return because you had a bad/old address on file, it won't matter to eBay. You'll still have to refund the buyer.
Actually, re-reading your post it sounds like delivery to your old address failed so it's already being returned to the authenticator.
You may not have any options at this point. Contact CS at the bottom of this page. Type 'agent' in the chat box. https://www.ebay.com/help/selling/selling-tools/ebay-authenticity-guarantee?id=4644
There used to be an email address for sneaker authentication, but that was a while ago and I don't know if it's still active. You can do some searching and should be able to find it in other posts on this forum, on the selling board.
