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add_filter( 'woocommerce_billing_fields', 'custom_woocommerce_billing_fields' );

function custom_woocommerce_billing_fields( $fields ) {

   // Over-ride a single label
   $fields['billing_first_name']['label'] = 'Your label';
   
   // Over-ride a single required value
   $fields['billing_first_name']['required'] = false;

   // Over-ride the entire thing
   $fields['billing_postcode']	= array( 
      'label'          => __('Postcode', 'woothemes'), 
      'placeholder'    => __('Postcode', 'woothemes'), 
      'required'       => true, 
      'class'          => array('form-row-last update_totals_on_change') 
   );

   /**
    * You can over-ride -  billing_first_name, billing_last_name, billing_company, billing_address_1, billing_address_2, billing_city, billing_postcode, billing_country, billing_state, billing_email, billing_phone
    */
   
   return $fields;
}