There is no button. It raises automatically dependent on your speed. Not sure what the setup is on a 2008 Gallardo, but on my 2004 car it raises to two different heights depending on your speed (not sure exactly what speeds they are - I'd need to check).

But take a careful look in your rear mirror when going slow when the spoiler is down, and then check the view again as you increase speed. (I think the first raised position is at about 100 KPH (60 MPH), and then it drops back at about 80 KPH. It changes height quite rapidly.