How to change 1 char in the string?

Question

I have this code:

string str = \"valta is the best place in the World\";

I need to replace the first symbol. When I try this:

Answer 1

I made a Method to do this

    string test = "Paul";
    test = ReplaceAtIndex(0, 'M', test);

    (...)

    static string ReplaceAtIndex(int i, char value, string word)
    {
        char[] letters = word.ToCharArray();
        letters[i] = value;
        return string.Join("", letters);
    }