How to edit the registry's string value using c#?

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;


namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

 private void button1_Click(object sender, EventArgs e)
        {
            string runKey = "SOFTWARE\\myapp";
            Microsoft.Win32.RegistryKey mykey = Microsoft.Win32.Registry.LocalMachine.OpenSubKey(runKey,true);
            string days = textBox1.Text;
            int c = Convert.ToInt32(textBox1.Text);
            if (c <= 30)
            {
                mykey.SetValue("Name", days);
                label1.Text = "Added";
            }
            else
            {
                textBox1.Text = "Enter < 30 days";
                label1.Text = "";
            }
            
        }
 }
}

Explanation:
   If your i/p above 30 only it will accept otherwise show alert.