What should I tell the leader if I want to change departments within the company?

You need to talk to the head of the department you are interested in first, and tell him why you can play a more important role in this department, where are your advantages and where are your potentials. If the leader agrees, you can ask him what you should do next. He knows more about your company's personnel operation process. Then follow the instructions of this leader.

If the leader is not interested in you, you'd better not make a move.