You'll receive a notification from the carrier when your update is ready, but only if you're using iOS 6.1 or higher. Once installed, AMBER and Emergency alerts are automatically sent to your phone unless you switch them off via Settings.
However, should you be tired of Obama, just know that there's no way to switch off Presidential alerts. WEA messages are always free of charge, so you don't have to worry about going over your texting limit when notified that you need to get the hell out of dodge. Read more >>