Changing itemprop from logo to image on custom logo?

I assume you have problem with google validator about itemprop="logo". You can hook into the get_custom_header filter and alter the HTML structure:

add_filter( 'get_custom_logo', 'my_custom_logo' );
// Filter the output of logo to fix Googles Error about itemprop logo
function my_custom_logo() {
    $custom_logo_id = get_theme_mod( 'custom_logo' );
    $html = sprintf( '<a href="https://wordpress.stackexchange.com/questions/269388/%1$s" class="custom-logo-link" rel="home" itemprop="url">%2$s</a>',
            esc_url( home_url( "https://wordpress.stackexchange.com/" ) ),
            wp_get_attachment_image( $custom_logo_id, 'full', false, array(
                'class'    => 'custom-logo',
            ) )
        );
    return $html;   
}

Add the above code to your theme’s functions.php or follow the steps here to create a child theme and use this code in it’s functions.php file.

Edit

Based on @birgire’s comment, i wrote another function to filter the wp_get_attachment_image():

add_filter('wp_get_attachment_image', function ($attachment_id, $size , $icon , $attr) {
        // If the class is 'custom-logo', then change the itemprop to image
        if ($attr['class'] =='custom-logo') {
            $attr['itemprop'] = 'image';
        }
        return $attr;
},10,3);