override-checkout-fields

add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );
function custom_override_checkout_fields( $fields ) {
     $fields['billing']['billing_city']['label'] = 'City';
     $fields['billing']['billing_phone']['required'] = false;
     $fields['order']['order_comments']['placeholder'] = 'My new placeholder';
    //removing fields
     unset($fields['order']['order_comments']);
    //adding custom fields
     $fields['shipping']['shipping_phone'] = array(
        'label'     => __('Phone', 'woocommerce'),
    'placeholder'   => _x('Phone', 'placeholder', 'woocommerce'),
    'required'  => false,
    'class'     => array('form-row-wide'),
    'clear'     => true
     );
     return $fields;
}