How would edit the default value on and existing array - have managed to edit the label using this but cannot get the options property to work using this:
add_filter( 'woocommerce_checkout_fields', 'custom_override_checkout_fields' );
function custom_override_checkout_fields( $fields ) {
$fields['billing']['billing_country']['options value="default"'] = 'My new select prompt';
$fields['billing']['billing_country']['label'] = 'Are you purchasing as a Company Y/N or from the UK?';
return $fields;
}