If you know what line to delete...
Say, for example, you got this message from SSH:
Offending key in /home/alexey/.ssh/known_hosts: 6
and you know what it means and you know why it's there and you want to remove it.
First, you can open your favourite text editor:
$ vim .ssh/known_hosts
and then, type this to Go to the 6th line, Delete it, and quit with saving (ZZ):
But there's an easier way!
just use sed:
$ sed -i 6d .ssh/known_hosts
to edit file In-place by Deleting 6th line.
Much less typing, don't you think?