Overide a CSS already loaded

by

Create a Child Theme. Add this to the styles.css file in the child theme:

 header > .logo {
    background: url("https://www.example.com/path/to/the/new/image.png") no-repeat 0 0 !important;
    }

Since the Child Theme CSS will load after the parent theme CSS, this should override the first setting, even if it is also set to !important .