<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<select>
<option>Все города</option>
<option>Москва</option>
<option>Казань</option>
<option>Сочи</option>
</select>
<div>Инфо о Москве</div>
<div>Инфо о Казани</div>
<div>Инфо о Сочи</div>
<script>
var div = document.querySelectorAll('div');
document.querySelector('select').onchange=function(){
var n = this.selectedIndex;
[].forEach.call(div, function(el, i){
el.style.display = n && i !== n-1 ? 'none':'block';
});
};
</script>
</body>
</html>