public class Test{
	public static void main(String[] args){
		String guess = "Colin is so great!";
		System.out.println(guess);
		guess = setCharAt(guess,1,'c');
		guess = setCharAt(guess,8,'X');
		System.out.print(guess);
	}
	public static String setCharAt(String s, int i, char c){
		return s.substring(0,i) + c + s.substring(i+1);
	}
}


makes output:

Colin is so great!
Cclin isXso great!