How to Update OptionSet using HttpClient requests

using (var httpClient = SetupHTTPClient())
            {
                var req = httpClient.GetAsync("EntityDefinitions(LogicalName='account')/Attributes(LogicalName='paymenttermscode')/Microsoft.Dynamics.CRM.PicklistAttributeMetadata/OptionSet?$select=Options").Result;
                JObject termCodeOpts = JsonConvert.DeserializeObject<JObject>(req.Content.ReadAsStringAsync().Result);

                if (termCodeOpts["Options"].HasValues)
                {
                    var optionList = termCodeOpts["Options"].ToList();
                    var labelList = optionList.Where(x => x.SelectToken("Label.LocalizedLabels[0].Label").ToString().Equals(termCode)).FirstOrDefault();
                    if (labelList != null)
                    {
                        return labelList["Value"].ToString();
                    }
                }
            }