You can extend your visa and have a multiple entries, however, generally speaking, it is hard to get a multiple-entries visa within China if you can't provide sufficient certificates. You can go to the Exit & Enter Department under Guangzhou Public Security Bureau to apply for it, it costs 160 RMB for extension of your visa's stay duration, 425 RMB for multiple-entry visa valid for 6 months, 635 RMB for multiple-entry visa valid for 12 months.