Header problem

I am a newbie to blogging and wordpress in general however I have read through the documentation. I have a simple question. I am trying to slightly modify my (default) theme and I am replacing the Header image with one of my own. Unfortunately, My Header only appears behind the default header. the page is at http://blog.life-exchange.com/. Please explain to me how to remove the default header as I add my new one. As you might have guessed my html/css skills are pretty limited. Thanks for your help!

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