First make a back of your entire site and the database before you start...

After this change the stylesheet.css
#mainWrapper {
background: none repeat scroll 0 0 transparent;
position: relative;
text-align: left;
top: 0;
vertical-align: top;
width: 1200px;
}

  1. change 1200 to 925px
  2. notice how the body is still too wide