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

Merged Chuck Norris's answer w/ Paulo Mendonça's using extensions methods:

/// 

/// Replace a string char at index with another char
/// 
/// string to be replaced
/// position of the char to be replaced
/// replacement char
public static string ReplaceAtIndex(this string text, int index, char c)
{
    var stringBuilder = new StringBuilder(text);
    stringBuilder[index] = c;
    return stringBuilder.ToString();
}