Hi Jeffery,
I don't think you will be able to make it. If you see the user guide (and the ws human task standard),
http://docs.jboss.org/jbpm/v5.1/userguide/ch.Human_Tasks.html#d0e3368
Into the human task life cycle there is no way to re open a task.
I think there should be a new task to modify data from the complete task.
Demian