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 keypair going into my desktop by this:

mkdir ~/.ssh
chmod 700 ~/.ssh
ssh-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 answerhere)

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ 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 ab:cd:ef:gh Please contact your system administrator. Add correct host key in /home/user/.ssh/known_hosts to get rid of this message. Offending key in /home/user/.ssh/known_hosts:1 RSA host key for user.server has changed and you have requested strict checking. Host key verification failed.

Solution

ssh-keygen -R hostname

This deletes the offending key from the known_hosts

The man page entry reads:

-R hostname Removes all keys belonging to hostname from a known_hosts file. This option is useful to delete hashed hosts (see the -H option above).