Hola Ugho,
mira a ver si algo así te sirve
<script language="javascript">
function cambia(donde)
{
valor=donde.texto.value
donde.selector.options[4].text=valor
donde.selector.options[4].value=valor
}
</script>
</head>
<body bgcolor="#FFFFFF" text="#000000">
<form name="form1" method="post" action="">
<input type="text" name="texto">
<select name="selector">
<option value="uno">uno</option>
<option value="dos">dos</option>
<option value="tres">tres</option>
<option value="cuatro">cuatro</option>
<option value="otro">otro</option>
</select>
<input type="button" name="Submit" value="Botón" onClick="cambia(this.form)">
</form>
