How to remove h1 tag from Drupal Garland theme in the header
if ($logo || $site_title) {
        print '&lt;h1&gt;&lt;a href="'. check_url($front_page) .'" title="'. $site_title .'"&gt;';
        if ($logo) {
          print '&lt;img src="'. check_url($logo) .'" alt="'. $site_title .'" id="logo" /&gt;';
        }
        print $site_html .'&lt;/a&gt;&lt;/h1&gt;';
      }


These are the lines 39-45 in Garland Themes page.tpl.php. 
This is probably the h1 tag you're talking about. If you want to change it to div it should look like this afterwards:

if ($logo || $site_title) {
        print '&lt;div class="youclassname"&gt;&lt;a href="'. check_url($front_page) .'" title="'. $site_title .'"&gt;';
        if ($logo) {
          print '&lt;img src="'. check_url($logo) .'" alt="'. $site_title .'" id="logo" /&gt;';
        }
        print $site_html .'&lt;/a&gt;&lt;/div&gt;';
      }