To make the correction, simply swipe left or right on the numbers displayed at the top of the calculator to remove a single number at a time.
You can do a few other things with the Calculator app. To copy a sum that you've calculated, simply press a finger over the number displayed on the calculator until the "Copy" option pops up. And in case you've never turned your iPhone to landscape mode while using the Calculator app, give it a try. You'll see a lot more functions to work with.