Task Resubmit

Hi Everyone:
During the development,I meet a problem.When a task has been assigned to a user,I want to reassign this task to another user,first,I cancel the task,this can be done by using the API(cancelTask()) that is included in the document.second I want to reassign the task to another user,I can not find the corresponding API in the doc. Does anyone give me some suggestion? or another way to complete such work?
Thanks in advance.