If an order has been processed, the best way to edit that invoice is to trigger it manually by using the actions dropdown to resend the invoice.
This is in the top center right of the return's dashboard.
Once you select "resent invoice" it will give you the option to edit the email address. Instead of sending this to the customer, you will instead put your own email there!
Then you can click on the link in your email to "complete your order"
And that will trigger the draft order to generate in Shopify.
You can use the hyperlink in the Redo Dashboard activity log for this return to access that draft order and either edit or mark it as paid.