WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED

There is a wrong line (line number 1)  in /home/user/.ssh/known_hosts file. Please remove that line from 

/home/user/.ssh/known_hosts file.

then try ssh again you can use "vim known_hosts " text editor to edit that file like

#vim /home/user/.ssh/known_hosts    

then type ":1" to goto line 1 and 

type "d" to delete that line. then save and close that file by typing ":wq"