China Z visa info need it

I want to covert my z visa into z visa from A company to B company,I want to know for apply z visa ?
I want to know how to apply z visa invitation letter,working card,alien card?And what kind of documents need it from employer & applicant ? I want to convert my z visa without leaving china.Any consultant company which is In dongguan and willing to handle this matter please contact me ( gmail.com|manuleo11) .It's urgent, kindly contact me ASAP .Thanks Manu