for (int i = 0; i <= answer.Length; i++) //answer is a string "theword"
            {
                if (answer[i] == passMe) //passMe is "A" for example
                {
                    hiddenWord = hiddenWord.Remove(i, 1);
                    hiddenWord = hiddenWord.Insert(i, passMe);
                }
            }

if (answer[i] == passMe[0])

for (int i = 0; i < answer.Length; i++)

for (int i = 0; i <= answer.Length; i++) //answer is a string "theword"
            {
                if (answer[i] == passMe[0]) //passMe is "A" for example
                {
                    hiddenWord = hiddenWord.Remove(i, 1);
                    hiddenWord = hiddenWord.Insert(i, passMe);
                }
            }

hiddenWord = new String(hiddenWord.Zip(answer, (a, b) => b==passMe[0] ? b : a).ToArray());