1) You can go to plugins/system/jat3/base-themes/default/blocks/header.php file and change this::
<h1 class=”logo”>
<a href=”index.php” title=”<?php echo $siteName; ?>”><span><?php echo $siteName; ?></span></a>
<h1>
to
<div class=”logo”>
<a href=”index.php” title=”<?php echo $siteName; ?>”><span><?php echo $siteName; ?></span></a>
<div>
2) Then, You can go to template.css file located in css folder of specific template and change all references of h1 tag here to div
h1.logo, div.logo-text h1 { font-size: 250%; line-height: 1; margin-left: 10px;}
/* Logo Image —*/
h1.logo { width: 210px; height: 30px; }
h1.logo a {
background: url(../images/logo.gif) no-repeat left;
display: block;
width: 210px;
height: 30px;
}