I don't know if you've discovered this on your own. When I have to change an assignment's due date, I go to the course I've made (10th Grade: Multiple Courses, in this case), click Assignments, click Manage, click the three dots for the item, click Edit, click the due date, select a new date, click Edit, and confirm that I want the change.

I've made a little picture to go with that. I don't know if it'll show up in this response.