Edit in JSFiddle

var i, matches, matches2;

matches = document.querySelectorAll('div.note, div.info');
for (i = 0; i < matches.length; i++) {
    matches[i].innerHTML = 'UPDATED with querySelectorAll';
}

matches2 = document.querySelector('div.alert'); // updates only first item
matches2.innerHTML = 'UPDATED with querySelector';

<div class="note">note text</div>
<div class="alert">alert text</div>
<div class="info">info text</div>
<div class="note">note text</div>
<div class="alert">alert text</div>
<div class="alert">alert text</div>
<div class="note">note text</div>
<div class="info">info text</div>