Stream read help

I want to make I thing where this program takes a .txt and replaces letter with numbes. I know how to do this with a text box (.replace) but how to I do it to a txt file

Read your file with File.ReadAllText(string path) and then you can do FileWriteAllText(string path, string content)

This would be better

Code:

---

using System;
using System.IO;

class Program
{
    static void Main()
    {
        //
        // It will free resources on its own.
        //
        string line;
        using (StreamReader reader = new StreamReader("file.txt"))
        {
            line = reader.ReadLine();
        }
        Console.WriteLine(line);
    }
}

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