From within the user's Compliance record, click into the requirement you would like to change the due date for. 

At the top of the requirement, you will have the option to Change due date. 

Note - altering the due date for a requirement with multiple workflow steps will update the due date for all outstanding steps in the workflow. 

If you do not see the Change due date option at the top of the requirement screen then you do not have permission to change the due date for this requirement.