In Normal view, when you tap a racer box, the box will turn yellow for 5 seconds. When you tap the box while it's yellow, a panel will pop up giving you an option to cancel the tap, or to swap the tap to another racer. This covers the two most common mistaps - an inadvertent tap, and tapping the wrong racer. If you choose to cancel, the app will ask you to confirm. If you choose to Swap racer, you'd tap the racer box you originally intended to, and the time stamp will be reassigned to that racer.
If you miss the 5-second window when the box is yellow to make a correction, tap the Fast-tap view toolbar button, and you can find the most recently tapped entry as the last item on the fast-tap list.
In Fast-tap view, you can cancel or swap any entry at any time during the race, simply by tapping the yellow Sequence # field. A panel will pop up giving you an option to cancel the entry or to swap the racer assigned to that time stamp.
In Keypad view, tap the yellow Sequence # field to cancel an entry. To swap a racer assigned to that time stamp, simply tap the yellow Bib field and edit the assigned bib number.