1. Go to the notifications list (swipe down from the top, or just turn on the iPad)
2. Give the notification a little swipe to the left, an "x" should appear to the right of the item (swiping right won't work)
3. Tap on the "x", it should go away.
Coming from Android or another system I can see how it wouldn't be obvious, you can't just swipe to dismiss, and as far as I know, there's no dismiss all button.