Note: please put this code in functions.php of your current theme.

  function custom_wpcsr_inc_vat_label_callback(){
    // Change the Exclude Val label
    $label = 'Your exc. Vat Label here';
    return $label;
  }
  add_filter( 'wpcsr_inc_vat_label', 'custom_wpcsr_inc_vat_label_callback' );
  function custom_wpcsr_exc_vat_label_callback(){
    // Change the Include Val label
    $label = 'Your inc. Vat Label here';
    return $label;
  }
  add_filter( 'wpcsr_exc_vat_label', 'custom_wpcsr_exc_vat_label_callback' );