If you wish to replace your site logo with something else, then you simply go to the toolbar, click, 'Design' --> 'Page Master' then click on your site logo and delete it. You can then add a new logo (.jpg, .gif or .png) from your own library (My Files) or software library. If you need to you can upload an image to your library and then add it to the page master.
Once added to the page master you a free to to scale, rotate or position the logo as you wish.
Just remember that the original or master image is in your own library (My Files) so don't delete your logo from this folder.