edit header.php (?) and remove the full (from <div id="topnav"> to </div><!-- end #topnav --> both inclusive)
edit style.css;
find:
#header {
width: 960px;
height: 110px;
margin: 0 auto 0;
padding: 0;
}
increase the height to the height of your new header image.
still in style.css, find:
.header-image #header #title-area {
background: url(images/logo.jpg) left top no-repeat;
}
that is the style for your header background image.
either enter a new header image name;
or, if you keep the same image name, simply edit/exchange the logo.jpg in the images folder of the theme.
don't forget to make backup copies of the theme files before editing(untested)