How do you extend a Z visa for 30 day stay after September 1?

I am Canadian with a Z. I would like to extend my stay for 30 days that expires September 11.

I have a foreign expert certificate. I am told that I need this to be cancelled by my employer. Apparently, they can do this online.

Please let me know what documents I need, and any regulations I need to know about. If there is a good agent you can recommend, please do.

b) all