I've done that recently, it's actually much simpler than it would seem. I recommend you read the iPad programming guide from Apple, it's about 100 pages in PDF (and you don't need to read all of it). Basically, you need to:
- run a command in Xcode that automatically converts your project to Universal
- add support for orientation changes, if you haven't done that earlier (it's required on iPad)
- go through the app, see what's broken and fix it (e.g. it's likely that you'll see some places where you need to fix autoresize settings for controls)
That's of course if you don't want to redesign the UI for iPad, which you'll probably want to do in the end (e.g. use split views, popup dialogs and various modals, and do less full screen view transitions). The UI that you'll get by going through this steps won't feel 100% iPad-y, but it will work, and will look much better than an iPhone-only app zoomed in, so it's a good start.