Demo by Juno_okyo

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title>Juno_okyo</title>
</head>
<body>
    <form action="javascript:void(0);" method="post">
        <select name="test">
            <option value="a" selected>A</option>
            <option value="b">B</option>
            <option value="c">C</option>
            <option value="d">D</option>
        </select>
        <input type="text" name="data">
    </form>

    <script>
    var form = document.forms[0];
    form.test.onchange = function() {
        form.data.value = this.value.toUpperCase();
    }
    </script>
</body>
</html>