I'm now in China. A company wants to hire me but I have only the L visa. What must we do to change to the Z visa?

You can sign the employment firstly, and then ask your emplyer to provide you a Visa Notification Form issued by Chinese authorized unit and Working Permit issued to foreign experts by The State Administration of Foreign Experts Affairs, Alien Employment License of the People's Republic of China issued by Ministry of Labor and Social Security, Letter of Invitation for Foreigners Engaged in Offshore Petroleum Operations in the People's Republic of China, etc.
Then change your L visa into a Z visa from the Public Security Bureau (PSM) Exit and Entry Administration office for the municipality.