As long as you haven't actually mailed or efiled, you can make changes. Some people pay in advance early on for various reasons, so there's no problem with having paid.
Are you still able to open your return back up? If TurboTax thinks you are finished, there will no longer be an orange "Take me to my return" button in the Tax Timeline.
If you can't open your return by the usual method from the Tax Timeline, report back, and we can tell you how to re-open it.