psfdek,
http://learn.javascript.ru/attribute...трибуты
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
</head>
<body>
<select id="ranks" name="ranks" size="5" multiple>
<option value="Koterov">one</option>
<option value="Wyke">two</option>
<option value="Wyke">two</option>
<option value="Wyke">two</option>
<option value="Wyke">two</option>
<option value="Wyke">two</option>
</select>
<script>
document.querySelector('#ranks').removeAttribute('multiple')
</script>
</body>
</html>