Si solo queres cambiar el atributo o agregar uno nuevo podes usar setAttribute().
https://developer.mozilla.org/es/docs/Web/API/Element/setAttribute
Código (javascript) [Seleccionar]
var select = document.getElementById("eleccion"),
option = select.getElementsByTagName("option"),
option2 = option[1];
option2.setAttribute('selected', 'selected');
https://developer.mozilla.org/es/docs/Web/API/Element/setAttribute