First of all, the 36mm nut on top of the headset should be properly adjusted (too tight is no good, too loose the front wheel will have up and down free play). Now open the folding box, tighten the screw at the center. The nut located at the side of the box should be adjusted, when the buckle is pressed down the folding box should be secure having no movement at all. Sorry for typing in English as I don't have a keyboard to type Chinese.
最好不要更換,應該能夠栓緊. 摺疊盒板手下的螺要調整, when the buckle is pressed down, the folding box should be tightly closed. Follow the instruction I mentioned in the previous post. You can also go to youtube to look for the relevant info. If you can tell me exactly which part is loose (photo if possible), then I will tell you what to do.