How to establish ssh key pair when "Host key verification failed"

I have set up ssh key pairs between my desktop and two servers, and from the servers to my desktop, but after reinstalling the OS on my desktop, I can't re-establish the key-pair going into my desktop by this:

mkdir ~/.sshchmod 700 ~/.sshssh-keygen -t ssh-copy-id username@server

I get the following error:

(names in italics changed to protect the innocent My desktop is Ubuntu, and I can't find the answer here)

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 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-middleattack)!It is also possible that the RSA host key has just been changed.The fingerprint for the RSA key sent by the remote host isab:cd:ef:ghPlease contact your system administrator.Add correct host key in /home/user/.ssh/known_hosts to getrid of this message.Offending key in /home/user/.ssh/known_hosts:1RSA host key for user.server has changed and you have requested strictchecking.Host key verification failed.