WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!

Let's deal with the following error:

$ ssh username@servername  

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@   @ 

WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! 

@   @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 

IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! 

Someone could be eavesdropping on you right now (man-in-the-middle attack)! 

It is also possible that the RSA host key has just been changed. 

The fingerprint for the RSA key sent by the remote host is 

23:00:20:83:de:02:95:f1:e3:34:be:57:3f:cf:2c:e7. 

Please contact your system administrator. 

Add correct host key in /home/xahria/.ssh/known_hosts to get rid of this message. 

Offending key in /home/xahria/.ssh/known_hosts:8 

RSA host key for localhost has changed and you have requested strict checking. 

Host key verification failed. 

to solve this problem go to /.ssh

$ cd /.ssh

$ nedit known_host

Delete all occurencies of servername and save the file.

That's it. Try again 
$ ssh username@servername