Change header image width

Hi, I’d like to change the default size of the header image. The Theme recommends 300px and I’d like it to be 1200px.

I’ve a child theme. I’ve been looking at the original functions.php and found this (only quoting relevant lines!):

add_action( 'after_setup_theme', 'path_theme_setup' );
function path_theme_setup() {
	$path_header_args = array(
	'flex-height' => true,
	'height' => 99,
	'flex-width' => true,
	'width' => 300,
	'header-text' => false,
	);
}

I tried changing the values in the child’s functions.php with no success (adding another hookup ‘after_theme_setup’), I even tried changing the values in the original functions.php, at least to see what happens, but the behavior continue the same…

Any ideas?? Thank you!!!