You should be able to change that in the shipping info during checkout before you pay.
It depends on your saved addresses - you can change either (or both) your registered address details (including phone number), or your delivery address details if it is listed separately from your registered address.
ebay has moved some things around so it's not always easy to get to the right pages, but you should find it by going to My eBay > Account > Addresses (with a potential detour in between Account and Addresses to "Personal information").
Ultimately you should arrive at a page that looks like this:
This link should take you directly to the same page: https://accountsettings.ebay.com.au/uas/addresses
