How to Change SharePoint Online masterpage from Seattle to Oslo using CSOM

using Microsoft.SharePoint.Client;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Security;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace UpdateMasterPage
{
public partial class UpdateMasterPage: System.Windows.Forms.Form
{
    public UpdateMasterPage ()
    {
        InitializeComponent();
    }


    private SecureString ConvertToSecureString(string password)
    {
        if (password == null)
            throw new ArgumentNullException("password");

        var securePassword = new SecureString();

        foreach (char c in password)
            securePassword.AppendChar(c);

        securePassword.MakeReadOnly();
        return securePassword;
    }
    private void button1_Click(object sender, EventArgs e)
    {
        Microsoft.SharePoint.Client.ClientContext cc = new Microsoft.SharePoint.Client.ClientContext("https://Gowtha.pnpdemo.com/");
        SecureString secstring = ConvertToSecureString(“********”);
        SharePointOnlineCredentials creds = new SharePointOnlineCredentials("gowtham@pnp.onmicrosoft.com", secstring);


        // The SharePoint web at the URL.
        Web web = cc.Web;

        // We want to retrieve the web's properties.
        cc.Load(web);

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

        var a = web.Title; // For debug checks.
        var master = web.CustomMasterUrl; // For debug checks.
        web.CustomMasterUrl = "/_catalogs/masterpage/seattle.master"; // Or the original page desired.
        web.MasterUrl = "/_catalogs/masterpage/seattle.master";
        web.Update();
        // Execute the query to server.
        cc.ExecuteQuery();
    }
}