I have a current China Visa with 6 months validity, need it more than 6 months

Hi, I'm holding 12 months China Visa with multiple entries valid from Oct 26, 2017 till 2018. However, I need it to be extended (even it's still valid for the next few months) due to I want to sell my car in China which requires China entry permit to be valid for more than 6 months. How can I do this with what documents needed handy?