Conversion of L visa into Z visa

You can't convert a L visa to a Z visa in China. I'm afraid you will have to go back to your country to apply for a China Z visa unless your Chinese employer promised to do it for you. The documents for applying a Z visa are:
1. application form that should be filled appropriately and authentically
2. passport valid for at least six months with at least 2 blank visa pages
3. One recent passport size photograph
4. photocopy of the passport information page
5. A Invitation Letter from a duly authorized Chinese organization, namely, Ministries and Commissions of the State Council, the Government of all Provinces, Autonomous Regions, and Municipalities directly under the Central Government, and Foreign Affairs Offices and Bureaus of Economy and Commerce thereunder
6. Covering letter from you addressed to the visa office
7. Alien Employment License issued by the Ministry of Labor & Social Security
8. Physical Examination Record for Foreigner issued by doctors from an Indian state hospital