Add this line to the head section of your template index.php file:

<?php $this->setGenerator('your company name or generator text'); ?>

Here is an example of how it will look for this site:

<head>
<jdoc:include type="head" />
<?php $this->setGenerator('Beanbox Company'); ?>
<link rel="stylesheet" href="/templates/<?php echo $this->template ?>/css/template.css" type="text/css" />
</head>

Save the file.