Man sell the bike to one of the many, many Chinese shorter than you and get another bike. It's not worth fussing around with a bike frame that doesn't fit you. No way, no how should a six foot tall man be riding around on a 50cm frame. That would be too small for my 5 foot 4 inch wife. You must look like a circus bear on one of those trick tiny bikes! Surely there is some shop in all of China that would be eager to sell you a larger bicycle.