In my opinion you should more look into task life cycle methods (as defined by WSHT spec) rather than modifying task instances. For instance you could use forward or delegate methods (depending on your needs) and then setDocumentContent to modify task content (variables) of the task.
Thank you for your reply ,you are right i need more look into the task life cycle but in my case changing task status is not enough.
Administrator can exit the current task(human task 2 or human task 3) and restart the completed task which is human task 1 in the image.forward or delegate is not enough for this.
The image is not clear so first task is human task 1 and second task is human task 2 and third task is human task 3