Can I change a user’s test score?

You can't arbitrarily change a score. But you CAN edit your question and re-score the test to adjust your test taker's score. Typically if you don't want to include the question in the score, edit the question and change the scoring method to 'no score'. Or, adjust the zero points possible answer to whatever points you now want to give for that question.

If you only want this to impact the one test taker (and no future test takers) - revert your changes made to the question after you do the re-score.