After you create an assignment, you can edit its start and due date, annotation task, or Guided Reading Mode settings by opening the Edit Assignment wizard. To do this, go to your Assignments page, find your assignment, and click the pencil icon (which will be replaced with "Edit Assignment" on narrow/mobile screens).
From there, you can make edits to your assignment settings and click Save. Students who are currently looking at the assignment will need to refresh the assignment to see updates.
Note: Editing a start date affects whether students can see the assignment or not. Students cannot see assignments with start dates in the future.