Hi, if you look in the stylesheet.css file there should be some code like this
#page{
width: 988px;
float: left;
background: #000000 url(<code>images/header.gif</code>) no-repeat;
color: #fff;
make a new header.gif that is the same size as the old one (you should find it in the images folder of your directory) and then replace it with a new one.