I'm going to Gejiu, Yunnan on September 20th. I don't know what the weather is like there What should I prepare?

No one can say for sure. The probability that the weather forecast can accurately predict the weather after 13 days is too small, especially the weather with little change. Basically, the old weather is suitable, but the temperature difference between morning and evening is still a little big, but it is not special. The key depends on where you come from and what kind of climate you adapt to. Personally, I don't need to wear a shirt during the day and a coat at night (provided that I go out to play at night).