program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;
using System.Xml;
using Microsoft.SharePoint.Client;
using System.Security;
using System.IO;
using System.Web.Script.Serialization;

namespace RestCallConsoleApp
{
    class Program
    {

        static void Main(string[] args)
        {
            try
            {
                SharePointOnlineCredentials cred = EnterCredentails();

                UpdateListItem(cred);
            }
            catch (Exception ex)
            {

            }
        }

        public static string GetFormDigest(SharePointOnlineCredentials cred)
        {
            string formDigest = null;

            string resourceUrl = "https://personal344.sharepoint.com/_api/contextinfo";
            HttpWebRequest wreq = HttpWebRequest.Create(resourceUrl) as HttpWebRequest;
            wreq.Credentials = cred;
            wreq.UserAgent = "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)";
            wreq.Method = "POST";
            wreq.Accept = "application/json;odata=verbose";
            wreq.ContentLength = 0;
            wreq.ContentType = "application/json";
            wreq.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f");
            string result;
            WebResponse wresp = wreq.GetResponse();

            using (StreamReader sr = new StreamReader(wresp.GetResponseStream()))
            {
                result = sr.ReadToEnd();
            }

            var jss = new JavaScriptSerializer();
            var val = jss.Deserialize<Dictionary<string, object>>(result);
            var d = val["d"] as Dictionary<string, object>;
            var wi = d["GetContextWebInformation"] as Dictionary<string, object>;
            formDigest = wi["FormDigestValue"].ToString();

            return formDigest;

        }

        public static void UpdateListItem(SharePointOnlineCredentials cred)
        {
            string result = string.Empty;

            Uri uri = new Uri("https://personal344.sharepoint.com/_api/web/lists/getbytitle('Test')/items(1)");
            HttpWebRequest wreq = (HttpWebRequest)WebRequest.Create(uri);

            wreq.Credentials = cred;

            string formDigest = GetFormDigest(cred);

            wreq.Method = "POST";
            wreq.Accept = "application/json; odata=verbose";
            wreq.ContentType = "application/json; odata=verbose";
            wreq.Headers.Add("X-HTTP-Method", "MERGE");
            wreq.Headers.Add("IF-MATCH", "*");
            wreq.Headers.Add("X-RequestDigest", formDigest);
            wreq.Headers.Add("Authorization", "BEARER" + formDigest);
            
            string stringData = "{'__metadata': { 'type': 'SP.Data.TestListItem' }, 'Title': 'I am updated Title now'}";
            wreq.ContentLength = stringData.Length;
            StreamWriter writer = new StreamWriter(wreq.GetRequestStream());
            writer.Write(stringData);
            writer.Flush();

            WebResponse wresp = wreq.GetResponse();
            using (StreamReader sr = new StreamReader(wresp.GetResponseStream()))
            {
                result = sr.ReadToEnd();
            }
        }

        public static SharePointOnlineCredentials EnterCredentails()
        {
            ConsoleColor defaultForeground = Console.ForegroundColor;

            Console.ForegroundColor = ConsoleColor.Green;
            Console.WriteLine("Enter your loginid:");
            Console.ForegroundColor = defaultForeground;
            string userName = Console.ReadLine();

            Console.ForegroundColor = ConsoleColor.Green;
            Console.WriteLine("Enter your password.");
            Console.ForegroundColor = defaultForeground;

            SecureString password = GetPassword();


            SharePointOnlineCredentials cred = new SharePointOnlineCredentials(userName, password);

            return cred;
        }

        private static SecureString GetPassword()
        {
            ConsoleKeyInfo info;

            //Get the user's password as a SecureString
            SecureString securePassword = new SecureString();
            do
            {
                info = Console.ReadKey(true);
                if (info.Key != ConsoleKey.Enter)
                {
                    securePassword.AppendChar(info.KeyChar);
                }
            }
            while (info.Key != ConsoleKey.Enter);
            return securePassword;
        }
    }
}