If the task has not expired:
You can still edit some characteristics of a task after you assign it. Select the task you would like to edit, then hit "Edit Dates" to update start and due date, the expiry date, question variation and allowing hints and videos.
If the task has already expired:
You will not be able to edit the task. Instead, you will have to reassign a new task. Select the task, then hit "Duplicate Task", make the changes and then save. You can then archive the original task that you no longer want by selecting it in the planner page and selecting the delete button.