How to Change the Calendar Type in the Regional Settings Using CSOM in SharePoint 2013 Online

Code Snippet: 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Security;
using System.Text;
using System.Threading.Tasks;
using Microsoft.SharePoint.Client;

namespace CSOMOffice365
{
    class Program
    {
        static void Main(string[] args)
        {

            string userName = "[email protected]";

            Console.WriteLine("Enter your password.");
            SecureString password = GetPassword();

            // ClienContext - Get the context for the SharePoint Online Site
            // SharePoint site URL - https://c986.sharepoint.com

            using (var clientContext = new ClientContext("https://c986.sharepoint.com"))
            {
                // SharePoint Online Credentials
                clientContext.Credentials = new SharePointOnlineCredentials(userName, password);

                // Get the SharePoint web
                Web web = clientContext.Web;

                // Regional Settings: Change the calendar type and alternate calendar type
                // Reference: https://msdn.microsoft.com/en-us/library/microsoft.sharepoint.client.regionalsettings.calendartype.aspx
                RegionalSettings regSet = web.RegionalSettings;

                // Change the calendar type to 5 - Korean Tangun Era
                regSet.CalendarType = 5;
                regSet.AlternateCalendarType = 5;

                // Update the regional settings
                regSet.Update();
                clientContext.Load(regSet);

                // Execute the query to the server
                clientContext.ExecuteQuery();

                // Display the updated calendar type
                Console.WriteLine("Calendar Type: " + regSet.CalendarType + "; Alernate Calendar Type: " + regSet.AlternateCalendarType);
                Console.ReadLine();
            }
        }

        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;
        }
    }
}

Result