Model/Api.php
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 (!$result) {
            throw new LocalizedException(__('Failed to create Dictionary item.'));
        }
    }
public function upsertDictionaryItem($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.'));
        }
    }
Upsert single Dictionary item

@param $dictionaryId
@param $itemKey
@param $itemValue
@throws LocalizedException