A result is wrong, but the winner has already played their next match. What now?

Once the winner has played their next match, you can't change the first result on its own, because the next match came out of it. So undo the later one first:

1. Open your competition's Results, press the Fix button on the later match, then reset result.
2. Then press the Fix button on the wrong match, then reset result.
3. Play the wrong match again, then the later one.

If the winner has gone further still, start from the latest of their matches and work back. If the draw is made again every round, every match played since has to be reset first.

See how do I fix a wrong score?