If your invoice is currently unpaid, you are able to modify the billing address as well as any invoice item.
Anyone with a URL to the invoice is now able to edit the Billing Address directly on the invoice. This URL can be found in the footer of the invoice if it is in PDF format.
When you are on the invoice page, simply click on Edit Billing Address, make the changes, and then click "Save Billing Address".
Below the invoice items, there is a link to "modify items". Please note that if the link is not there, your invoice is in a state where it is no longer possible to modify the items.
The "modify items" link will take you to the cart page where you can make changes to the items. Once finished, you can navigate back to the invoice via the "View Invoice for Cart" link at the bottom of the page.