In config.php:change define('SMTP_HOST','localhost'); to define(SMTP_HOST','mail.yourweb.com');
Note, I'm not sure if you absolutely have to change this specifically.
The SMTP mail server name of "localhost" may work as it is, or you may need to change it to specify a mail server typically in the format like "mail.yourweb.com" which will be whatever your host has advised you it is.
The other important variable is:
define('USE_SMTP',false);
Change it to read:
define('USE_SMTP',true);
There are other variables such as the user-name/password for your outgoing mail server if it requires authentication, and to change the port number. All these things, your host can tell you what is required.
define('SMTP_PORT',25);
define('SMTP_AUTH',false); //if your SMTP require authentification, set this to 'true'
define('SMTP_USERNAME','YOUR_SMTP_USERNAME');
define('SMTP_PASSWORD','YOUR_SMTP_PASSWORD');