Ubuntu Linux includes the OpenSSH client and server in its software repositories. OpenSSH is a set of open-source programs which implement the secure shell protocol. The SSH client securely accesses remote SSH servers, providing secure access to a Linux shell over the local network or Internet. The OpenSSH client program is installed by default on Ubuntu, but the OpenSSH server isn't. You can reinstall an OpenSSH package on your Ubuntu system if its files become corrupted. Ubuntu's software installer will overwrite the package's existing files on your system with the original files from the software package.
Click the "Applications" menu, click "Accessories" and click "Terminal."
Type "sudo apt-get --reinstall install package_name" into the terminal window, replacing "package_name" with "openssh-client" for the OpenSSH client or "openssh-server" for the OpenSSH server.
Type your password when prompted and press "Enter."