code: public function upsertDictionaryItem($dictionaryId, $itemKey, $itemValue)
      {
          $body = ['item_value' => $itemValue];
          $url = $this->_getApiServiceUri(). 'dictionary/'. $dictionaryId . '/item/' . urlencode($itemKey);
          $result = $this->_fetch($url, \Zend_Http_Client::PUT, $body);
  
          if (!$result) {
              throw new LocalizedException(__('Failed to create Dictionary item.'));
          }
      }
docs: Upsert single Dictionary item
  
  @param $dictionaryId
  @param $itemKey
  @param $itemValue
  @throws LocalizedException