How to remove the WordPress logo from login and register page?

I want to remove WordPress logo from login and register page. I think I have to make some changes in .css file for this. So I want to know in which css file do I have to make the changes?

You can add a filter to login_head. As explained in the Codex, add this to your code:

function my_custom_login_logo() {
    echo '<style type="text/css">
    h1 a {background-image:url(https://example.com/your-logo.png) !important; margin:0 auto;}
    </style>';
}
add_filter( 'login_head', 'my_custom_login_logo' );