Since our server admin has left it would be a good time to change the login key for root? How do we go about this?
You will need to generate a new keypair, add the generated public key to root's
The format of the $HOME/.ssh/authorized_keys is one key per line, if you have more than one key, you will have to identify the one currently in use to remove it.
Generate a new key with the ssh_keygen command
Now add the new public key (id_rsa.pub) to root's authorized_keys file, test that you can log in correctly with it and then remove the old key.
You may want to consider disabling roots ability to log in directly to your server and have people who need root access log in using their own credentials and then use sudo to perform admin tasks.