- Visit http://download.whmcs.com/
- Click the Download button under the Latest Stable Version.
- Unzip the contents of the zip file to a folder on your computer.
- Rename the file configuration.php.new to configuration.php.
- Upload the entire whmcs folder to your website.
- You can rename the folder to whatever you like (billing, clients, etc…).
- Enter your website’s cpanel and create a database and a user. Write down the database’s name, the username and the password.
- Visit the installation script at http://www.yourdomain.com/whmcs/install/install.php to run the installer process.
- Follow the instructions on screen to install. On step 3 use your license key and the database’s credentials from step 8.
- When complete, delete the install folder from your web server and CHMOD the configuration.php file back to 644.
The official documentation here.