First make sure that you have the registration for Sins (blue orb > add registration will take you to the screen that should list all your numbers). If you don't, you'll have to add one with your serial #. Then whenever an update is available it will show up in both an Updates tab (hidden if there's nothing), and in My Games your Sins listing will have a yellow ! icon and it will say update available. You can click on it and instead of the normal Run button on the bottom you'll have Update. Impulse will do the rest.