code: function renameCustomField($oldName, $newName)
      {
          $encodedOldName = urlencode(mb_convert_encoding($oldName, "UTF-8"));
          $encodedNewName = urlencode(mb_convert_encoding($newName, "UTF-8"));
          return $this->put("contacts/fields/custom/${encodedOldName}/${encodedNewName}");
      }
docs: Renames a custom contact field. The data type and the recorded values for
  the contacts remain unchanged.
  
  @param string $oldName
  the current name of the field
  @param string $newName
  the new name of the field
  @return \em com_maileon_api_MaileonAPIResult
  the result object of the API call
  @throws com_maileon_api_MaileonAPIException
  if there was a connection problem or a server error occurred