hi,
I don't think you can undo a completed task, basing on the task life cycle. http://docs.jboss.org/jbpm/v5.3/userguide/ch.human-tasks.html#d0e5129
But you can add an event or connection or any other method to re-enter this task node in the process. A new task instance will be created. You may need to manually 'rollback' what you have done to your business data before re-enter this task node.
Ted