There’s isn’t a way entirely using the UI. With the Twig Advanced Extension you can call arbitrary functions like update_field. I would only try this on a staging site first and I would probably opt to just display API data if you can. This has come up a number of times to the point we’ll eventually have a video going through this, although it won’t be for the faint of heart. Ideally we would have an “Action” system in Cornerstone, but we probably won’t tackle that till we start handling forms. Let us know if this helps.
{{ function('update_field', 'my_field', looper.item.apiValue, post.id) }}