If you would like to change the WordPress logo on the login page, here’s a quick solution.
Access your /wp-admin/ folder inside your WordPress installation. Go to folder /images/. Here, make a backup of logo-login.png file.
Now create your own logo file. It should be exactly 310×70 pixels and have a transparent background. Any image editor can do that, free ones included, so choose one.
Save the image to png and upload as /wp-admin/images/logo-login.png
The new image will be available almost instantly.