there are many buses going between Zhangjiajie and Fenghuang. You can take the bus at the Zhangjiajie Bus terminal for 3.5hours, cost about 80rmb. It’s rather frequent, almost 1 bus each hour
By Train:
you can train from Zhangjiajie Railway Station towards Jishou Station, then switch to direct bus to Fenghuang outside the railway station.
There are more buses in the afternoon, the last one leaves at 8:30pm