change the value of input text in a form inside of form using javascript
===

When I click on Button, I want the input text value change to 'hello', but it doesn't seem to work, here is my code. Pros please help I'm really noob, thanks in advance! 

    <script type="text/javascript">
    function selectfile(){
        document.uploadform.selectfile.value='hello';
    }
    </script>


    <body>
    <form name="uploadform" id="uploadform">
        <input type="text" name="selectfile" id="selectfile" value="Hi"/>
        <input name="upload_file" type="button" onClick="selectfile()" value="Button"/>
    </form>
    </body>