There is functionality to upload a logo to the login page. Of the top of my head I believe it is under Administration --> Globals.
As far as removing the OpenEMR logo from the front page, I believe that would require custom code (though again, I'm doing this from memory).
Inside the app I think the only reference to OpenEMR is on the about page and the logo on the top left of the main menu. There is no functionality to remove those items as far as I'm aware.