Update/Change custom TaxonomyFieldValue in document library list in sharepoint using C# CSOM - sharepoint

Question

list of documents thats contains custom taxonomy field column named subject.
Need to update subject of thousands records/documents.
Please any idea to update the taxonomy field such subject programtically using C# CSOM

Answer 1

Please try to use this method:
public void UpdateTaxonomyField(ClientContext ctx, List list,ListItem listItem,string fieldName,string fieldValue)
{
Field field = list.Fields.GetByInternalNameOrTitle(fieldName);
TaxonomyField txField = clientContext.CastTo<TaxonomyField>(field);
TaxonomyFieldValue termValue = new TaxonomyFieldValue();
string[] term = fieldValue.Split('|');
termValue.Label = term[0];
termValue.TermGuid = term[1];
termValue.WssId = -1;
txField.SetFieldValueByValue(listItem, termValue);
listItem.Update();
ctx.Load(listItem);
ctx.ExecuteQuery();
}

Answer 2

public static void UpdateListofLibraryHavingTaxonomyField()
{
string siteUrl = "http://abc:55555/sites/xyz/";
string libName = "Knowledge Repos";
string strTermGuid = "Your new/update term guid";
try
{
Dictionary<string, string> dictIdsSubjectsChange = ReadFromExcel();//Here to read all records (approx 2000 records) that we want to change
ClientContext clientContext = new ClientContext(siteUrl);
List list = clientContext.Web.Lists.GetByTitle(libName);
FieldCollection fields = list.Fields;
Field field = fields.GetByInternalNameOrTitle("Subject1");
ListItemCollection listItems = list.GetItems(CamlQuery.CreateAllItemsQuery());
clientContext.Load(listItems, items => items.Include(i => i["Subject1"], i => i["ID"]));
clientContext.Load(fields);
clientContext.Load(field);
clientContext.ExecuteQuery();
TaxonomyField txField = clientContext.CastTo<TaxonomyField>(field);
TaxonomyFieldValue termValue = null;
if (dictIdsSubjectsChange != null)
{
foreach (ListItem listItem in listItems)//Loop through all items of the document library
{
string strCurrentID = "0";
try
{
strCurrentID = listItem["ID"].ToString();
}
catch (Exception) { }
if (dictIdsSubjectsChange.ContainsKey(strCurrentID))//Checking to change ot not
{
termValue = new TaxonomyFieldValue();
termValue.Label = "Special Knowledge";
termValue.TermGuid = strTermGuid;
termValue.WssId = 246;
txField.SetFieldValueByValue(listItem, termValue);
listItem.Update();
clientContext.Load(listItem);
}
}
clientContext.ExecuteQuery();
}
}
catch (Exception ex) { }
}
}