Britton here with the Shopify support team.
At the present time there is no way to change how the "detailed description of contents" is pulled into the customs forms. If you wanted to edit what appears there you would need to make those changes of the product itself.
I hope that clears up the amibguity!
This is disappointing. A product title in the store has a completely different purpose from the description that goes on the customs form. To create a separate description for customs would simply involve one additional data field to fill out on the product page that could be ported into the field for the customs form. that would be super helpful. I am very tired of going in and manually changing the description for every item on every international order.