When you select "Adjust Sale," the status of the listing can be updated from "Completed" back to "Account Sale," and you will then be able to make the desired changes to the listing.
Once your changes are complete, you can change the status of the listing back to "Completed".
Select Adjust Sale
Select Undo Everything
Enter a Reason for the Adjustment & Continue
If the Account Sale status is to be taken back to Unconditional refer to the article: "Reversing an Account Sale Status"