If your visa is China tourist visa, you can go to PSB yourself or ask an agency to help you, documents required are as below:
1-passport;
2-passport photo;
3-temporary residence registration form;
4-a bank reference, tells you have RMB21000 or USD3000 in the bank account, the bank should be located in beijing.
If your visa is business/F visa, you'd better ask a visa agency to do the extension.
Documents required are as below:
1-passport;
2-passport photo;
3-temporary residence registratin form.
Hope you are clear what should you do, while any more query, you can send email to [email protected] or call 15010268301.
Maggie