We are going to Changzhuo soon and like to visit Detain waterfall.

I see. You refer to Chongzuo, right? Well, no diret bus goes there. I suggest you take a bus at Chongzuo International Passenger Transport Center to Daxin County first. Buses are available from 07:25 to 17:20 every 15min. It costs CNY36 and takes around 2h. Upon getting there, transfer to tourist bus to the waterfall in an hour. You can use the same way for back trip! One day is tight for around trip, considering the sightseeing. So you can consider stopping a night at Daxin.