Function modify a letter in a string Learn programming C#

using System;
public class exercise110
{
    public static void ChangeChar(ref string text, int position, char letter)
    {
        text = text.Remove(position, 1);
        text = text.Insert(position, letter.ToString());
    }

    public static void Main()
    {
        string sentence = "Tomato";
        ChangeChar(ref sentence, 5, 'a');
    }
}