You’re not blind, it’s not as straight forward as they could make it.
On an order after you’ve clicked “Buy Shipping”, towards the bottom you should see a link that says “Buy Shipping Preferences”, click that. From there you can edit your Ship From address.
I think the direct link is : https://sellercentral.amazon.com/gp/shipping/manager/shippingLocation.html/ref=ag_xx_cont_shipset
It’s also in Seller Central under Shipping Settings ……
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