Through the admin panel change the location with which Magento pulls the logo image.

System -> Configration-> Design -> Header

The best way to change the logo would be to upload one to your_magento_install_directory/skin/frontend/default/default/images and update the admin area with the filename of the logo you uploaded.

Or you could also just overwrite the logo.gif or image that your site is linking to with your own and not worry about the Magento admin area.