function myFunction() {
            var letters = document.getElementById('text');
            for (var i = 0; i < letters.innerHTML.length; i++) {
                //only change the one you want to
                if (i == 4) {
                    letters.innerHTML = letters.innerHTML.replace(letters[i], '<span style="color: yellow;">'+letters[i]+'</span>');
               }
            }
        }