Your question is bit unclear, What do you mean vy BPM screen? Is this screen created out of human task?
Which task you want re-assign? Is it a current task or any other task?
If you provide bit more information, it will be easy to give you right solution.
Otherwise standard approach to dynamically assign a task to particular user
- go to assign section of human task
- double click on default handler box
- in pop up screen choose name expression
- to add user click on green button and selection expression
- from your task payload select particular element which is having userid info.