Once a task has been assigned, you still have the option to edit it.
You have the option to edit the following:
Start date/Due date
To edit a task, navigate to the one you wish to edit using the Tasks tab or Planner. From there, click 'Edit task.'
Note: The due date must be in the future for you to be able to make any changes.
New Feature: Add or remove students from an assigned task
New Feature: If the task is assigned to multiple classes - you can now remove a class from the task