Edit the configuration file for SSH with the command:

vi /etc/ssh/sshd_config

Find the line that starts with PermitRootLogin and change the no to yes. You can find this line about 2 pages down from the top. Save the file by first pressing Ctrl-O and then Enter. Exit with Ctrl-X.
Restart the sshd service with the command:

service sshd restart

Note: Alternatively, use the command:

/etc/init.d/sshd restart