Terminal - ssh - REMOTE HOST IDENTIFICATION HAS CHANGED?

I am trying to do some command line networking, but when I type ssh root@"devices ip address" it comes up with the error:

lucas-computer:~ LukeB$ ssh root@10.0.1.6
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ 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
fd:07:34:3b:d1:76:1e:c2:e7:95:14:d3:b2:23:47:7f.
Please contact your system administrator.
Add correct host key in /Users/LukeB/.ssh/known_hosts to get rid of this message.
Offending key in /Users/LukeB/.ssh/known_hosts:4
RSA host key for 10.0.1.6 has changed and you have requested strict checking.
Host key verification failed.


I know for a fact nobody is "doing anything nasty" so am not worried, but when I go to add the key to known hosts I type: /Users/LukeB/.ssh/known_hosts
into terminal and receive the error:

lucas-computer:~ LukeB$ /Users/LukeB/.ssh/known_hosts
-bash: /Users/LukeB/.ssh/known_hosts: Permission denied

Why is this as I am a system admin?

Can anybody tell me very simply how to add the correct host key to known hosts?

Thanks