The answers here are terrible advice. You should never turn off StrictHostKeyChecking in any real-world system (e.g. it’s probably okay if you’re just playing on your own local home network – but for anything else don’t do it).
Instead use:
ssh-keygen -R hostname
That will force the known_hosts
file to be updated to remove the old key for just the one server that has updated its key.
Then when you use:
ssh user@hostname
It will ask you to confirm the fingerprint – as it would for any other “new” (i.e. previously unseen) server.