Once a request has been placed, it cannot be edited. To change a data or appearance option, edit and request the content again.
To do this go to My Content and find the content item you need to change. Click the More options menu to the right of the content item and select Edit and request.
You will then be taken to the content details page for that item. Here you can edit any customizations. When ready, request content as usual and it will be fulfilled as a new item.