How to task list update Assigned due date with the new Adjusted due date.

I have a task list / on the form and in the list I have  Assigned due date and Adjusted due date. I need the assigned due date to update with the  adjusted due date when the adjusted date date is not empty. 

so.....

Assigned due date == Adjusted due date IF  Adjusted due date is not blank

Any help would be great 

I have research the internet for days with no luck. 

Rule in list or on form or even a workflow............

Thank you