public string ReplaceLetters(string wordToSearch, char letterToFind, char letterToReplace)
{
    StringBuilder word = new StringBuilder(wordToSearch);

    int index = 0;

    foreach (char letter in wordToSearch)
    {
        if (letter.Equals(letterToFind))
        {
            word.Remove(index, 1);
            word.Insert(index, letterToReplace);
        }

        index++;
    }

    return word;
}