It's the same as in XP. Open Internet Explorer, click Tools > Windows Update, and it should quickly scan your machine, then tell you what updates are available. If SP1 is there, you obviously haven't got it yet, so select it, and everything from there on should be mostly automatic. Be aware, though, that SP1 can take up to an hour to download and install, so try to do it when you don't need to use the computer for a while.