Please find below code to change label & placeholder in woo-commerce checkout billing form

add_filter( ‘woocommerce_checkout_fields’ , ‘custom_override_checkout_fields’ );
function custom_override_checkout_fields( $fields ) {
unset($fields[‘billing’][‘billing_last_name’]);
$fields[‘billing’][‘billing_first_name’][‘placeholder’] = ‘Contact Name’;
$fields[‘billing’][‘billing_first_name’][‘label’] = ‘Contact Name’;
return $fields;
}