Hi,
add_filter( 'generate_logo', function() {
return 'www.mydomain.com/images/logo.jpg';
} );
or
add_filter( 'generate_logo', function() {
return '/images/logo.jpg';
} );
Should work fine.
I place the above code in functions.php file ?
Here’s how to add PHP – https://docs.generatepress.com/article/adding-php/
So to answer the question, yes you can place it on the bottom part of the functions.php of your child theme.
Placing it in the functions.php of the parent theme has its risks because any changes you’ve made there will be wiped when the plugin updates.