Example: update List Item

Example.UpdateListItem.cs

using (var client = new SPHttpClient(webUri, userName, password))
{
    var listTitle = "Tasks";
    var itemId = 1;
    var itemPayload = new {  __metadata = new { type = "SP.Data.TasksListItem" }, Title =  "Approval Task"};
    var endpointUrl = string.Format("{0}/_api/web/lists/getbytitle('{1}')/items({2})",webUri,listTitle,itemId);
    var headers = new Dictionary<string,string>();
    headers["IF-MATCH"] = "*";
    headers["X-HTTP-Method"] = "MERGE";
    client.ExecuteJson(endpointUrl,HttpMethod.Post,headers, itemPayload);
    Console.WriteLine("Task item has been updated");
}