Program.cs
 

using System;

namespace MyApplication
{
  class Program
  {
    static void Main(string[] args)
    {
      string txt = "Hello World";
      Console.WriteLine(txt.ToUpper());   // Outputs "HELLO WORLD"
      Console.WriteLine(txt.ToLower());   // Outputs "hello world"
    }
  }
}

Result:
HELLO WORLD
hello world