Replace Start With Finish

//Write a program that replaces all occurrences of the substring "start" with the substring "finish"
//in a text file. Ensure it will work with large files (e.g. 100 MB).

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;

class ReplaceStartWithFinish7
{
    static void Main()
    {
        string start = "start";
        string finish = "finish";
        StreamReader read = new StreamReader("../../input.txt");
        StreamWriter write = new StreamWriter("../../input2.txt");
        string line = "";

        using (read)
        {
            using (write)
            {
                line = read.ReadLine().ToLower();
                while (line != null)
                {
                    line = line.Replace(start, finish);
                    write.WriteLine(line);
                    line = read.ReadLine();
                }
            }
        }
        File.Replace("../../input2.txt", "../../input.txt", "../../backup.txt");
        // if you want to delete backup file uncomment next row
        //File.Delete("../../backup.txt");
    }
}