@JennieMcCulloch ...
The "Modules" screen is really used as a place to build out the structure of your course...adding things like Pages, Discussions, Assignments, and Quizzes. If you use the "Edit Assignment Dates" screen to bulk edit assignment due dates, any of your assignments that are already within your "Modules" page will be updated with the due date you set. So, while I understand why you'd want to update per module, I'm not sure it's totally necessary since you have access to edit all the dates via the "Assignments" index page. I hope this helps to clarify how things work in Canvas "Modules" and "Assignments".