I usually advise students with such a high score not to retake, especially as you have no plans to study engineering. What about stats, math, physics, chem, and the like? If those subject are in the cards, you could consider retaking to get a higher math sub score. BUT, you run the real risk of getting a lower composite score, which is not a risk I am sure is worth taking. Years ago, I read that up to 40% of high scorers will do worse on a retake, and that is not going to help your app.
I guess in your shoes I would consider your potential major. Other options could be trying the SAT 1 or 2, to show competency in math (31 is, of course, more than competitive.) Befor deciding though, check Harvard’s website and see if they require all test scores, or all sittings of the ACT or SAT.
Remember of course that the odds are not in your favor, so if yiu don’t get in, it’s highly unlikely that it will be due to a 31 in the math section of the ACT.