Edit characters in a String in C#

You can't since strings are immutable.

You could always roll your own extension method to do something similar:

public static string ReplaceAtIndex(this string original, 
    int index, char character)
{
    char[] temp = original.ToCharArray();
    temp[index] = character;
    return new String(temp);
}

And call it like:

string newString = myString.ReplaceAtIndex(0, 'd');

As Brian said, use a StringBuilder, instead:

string myString = "boom";
StringBuilder myStringBuilder = new StringBuilder(myString);
myStringBuilder[0] = 'd';
// Whatever else you're going to do to it

Whenever you need the string again, call myStringBuilder.ToString()