you have a default theme there..you can choose from many good free themes and then replace the header image in the image folder and then upload it
if you go to theme manager or something like that you have to look under the style.css sheet and find where it says the header image url and change it to the image url of the new logo
First of all I want to suggest you to change your theme..select the best one from here..: 20 More Free First-Class Wordpress Themes then after that you need to know where exactly the logo of the site would be located..for an example, the top left of the header or else..it depends from the themes you uploaded.. this is the video that show you how to customize your WP theme Quick & Easy Method to Customize a WordPress Theme and also you can also hack the Wordpress with different kind of tutorial or maybe some coding from this site called WebDesignerWall I hope that will help you..