Where to find the html.tpl.php $head variable content generation
The most common way to add information to the &lt;head&gt; is using  drupal_add_html_head().

If the data you want to alter was added using that function, you can alter it by implementing hook_html_head_alter() within a custom module. Using the alter hook requires that you know the $key of the data that was added during drupal_add_html_head().

If you want to alter CSS or JS, use hook_css_alter() and hook_js_alter() respectively.