Download and unzip the contents of the zip file to a folder on your computer.
You can find the latest version of WHMCS via your Client Area
You will now rename the file configuration.php.new to configuration.php
Afterwards, upload the entire WHMCS folder to your website (Where you wish to have this located)
ie: If you wish to have WHMCS accessible at http://www.domain.com/admin/ you will rename the folder to admin under public_html, and emsire your WHMCS files are in this folder.
Now visit the installation script at http://www.yourdomain.com/whmcs(or what you have named the folder)/install/install.php to run the installer process
Follow the instructions on screen to install which will involve setting file permissions as listed below, entering your license key and setting up your primary admin account
When complete, delete the install folder from your web server and change the permissions of the configuration.php file back to 644.
Required file & folders permissions
To change your file's permissions in cPanel under File Manager, you will simply right click the file / folder and click Change Permissions.
/configuration.php CHMOD 400 Readable
/attachments CHMOD 777 Writeable
/downloads CHMOD 777 Writeable
/templates_c CHMOD 777 Writeable
The above applies unless your php is suPHP or PHPSuExec. If using DSO as your php handler, you must use 644 permissions.