function change_why_box(box_id){

var box = 'why_box_' + box_id
var drop = 'why_drop_' + box_id

document.getElementById(drop).style.display = 'block'
document.getElementById(box).style.display = 'none'

var dropdown = document.getElementById('why_dropdown_' + box_id)
dropdown.selectedindex = 0

}

<tr id="row_1" border="0" class="" onMouseOver="this.className='mmrskift'" onMouseOut="check_mouse_out('row_1')"><td><center>34</center></td><td>Andreas Fredslund</td><td><input type='text' onkeydown="keye(event, 1)" onfocus="test('row_1')" id='field_1' name='26mm' size='7' onblur="document.getElementById('row_1').className=''" /></td><td>for</td><td width='200'><div id='why_drop_1' style='display:block'>
	<select id='why_dropdown_1' name='why_dropdown_1' onchange='test_why(1, this)'>
	<option value='1'>DT. HBO mål</option>
	<option value='2'>DT. tider</option>
	<option value='3'>Vundet konkurrence</option>
	<option value='4'>Kasse tider</option>
	<option value='5'>Nået mål</option>
	<option value='0'>Skriv selv grund</option>
	</select>
	</div><div id='why_box_1' style='display:none'><input type='text' onkeydown="keye(event, 1)" onfocus="test('row_1')" id='why_1' name='26why' size='25' onblur="document.getElementById('row_1').className=''" /> <a href='#' onclick='change_why_box(1)'> Dropdown</a></div></td></tr>