In Terminal, how do I add a host key to the file known_hosts?

In Terminal, how do I add a host key to the file known_hosts? Also, how do I remove a given host key from the same file?

I trying to ssh into a computer at my school and when I type in the command johndoe@mu.math.umn.edu, it returns:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    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 a host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
fb:59:b6:4f:e0:9d:99:64:12:44:c8:16:1c:46:b0:92.
Please contact your system administrator.
Add correct host key in /Users/johndoe/.ssh/known_hosts to get rid of this message.
Offending RSA key in /Users/johndoe/.ssh/known_hosts:1
RSA host key for mu.math.umn.edu has changed and you have requested strict checking.
Host key verification failed.

I am not sure, but I think want I want to do is add this "finger print for the RSA key" to the file known_hosts.