You can't change a confirmed score yourself, but your club can correct it. If levels have already been updated from the wrong score, they aren't recalculated, but the club can adjust your level by hand.
What to do
Contact your club.
Tell them which match it was and the correct score.
What the club can do
Correct the score at any time.
If it's corrected before levels are updated (about an hour after confirmation), levels are calculated from the correct score.
If levels were already updated, they aren't recalculated. The club can set any player's level to the right value.
Good to know
Before a score is confirmed, the other team can dispute it. See "How do I confirm or dispute a reported score?".
Nettla support can't change scores or levels. Ask your club.