If you want to change the due date, you can click on the task in the Planner, then click on Edit Dates. This is applicable for both custom and adaptive tasks.
Note: For custom tasks, this will only be possible if the task hasn't closed. Custom tasks will close as soon as the task reaches expiry.
Hint: If you suspect you might need to extend the due date of a task, then set the expiry date further in advance.