Consider the program:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main()
        {
            String str1;
            String str2;

            Console.Write("Enter String : ");
            str1 = Console.ReadLine();
            
            str2 = str1.Replace('i','I');
            
            Console.WriteLine("String after replace method : " + str2);
        }
    }
}

Output

Enter String : This is india
String after replace method : ThIs Is IndIa

Here, we replaced character "i" with the "I".