how i can go to three gorges dam from yichang city and come back in the same day

You can take public bus no. 4 to Yemingzhu Bus Station. Then transfer to bus no.8 to Three Gorges Scenic Area. It will take around 45 mins on the way.
Or you can just take a taxi to the scenic area directly. The distance from the city center to the destination is about 21 miles. The cab fare is about CNY 50.
The return way is the same. You have enough time to tour this scenic spot within one day.