PHPMailer default configuration SMTP

Create a function, and include / use it.

function create_phpmailer() {
  $mail             = new PHPMailer();
  $mail->IsSMTP(); // telling the class to use SMTP
  $mail->SMTPAuth   = true;                  // enable SMTP authentication
  $mail->Host       = "mail.yourdomain.com"; // sets the SMTP server
  $mail->Username   = "yourname@yourdomain"; // SMTP account username
  $mail->Password   = "yourpassword";        // SMTP account password
  return $mail;
}

And call create_phpmailer() to create a new PHPMailer object.

Or you can derive your own subclass, which sets the parameters:

class MyMailer extends PHPMailer {
  public function __construct() {
    parent::__construct();
    $this->IsSMTP(); // telling the class to use SMTP
    $this->SMTPAuth   = true;                  // enable SMTP authentication
    $this->Host       = "mail.yourdomain.com"; // sets the SMTP server
    $this->Username   = "yourname@yourdomain"; // SMTP account username
    $this->Password   = "yourpassword";        // SMTP account password
  }
}

and use new MyMailer().

Can I not just edit the class.phpmailer.php file?

It's best not to edit class files themselves because it makes the code harder to maintain.