How can I change a typo in the author affiliation after publication?

First of all, calm down. This is not as big of a mistake as you think it is. Many if not most published work contains insignificant mistakes such as minor typos. If it would make you feel better, I once had a typo in my own name (I blame autocorrection).

Contact the editor. They are probably going to publish a correction or erratum. If it is already published, they probably won't fix the original published article even if it is online only. (I am not familiar with the ACM Platform.)

Probably you cannot change it , usually journals inform authors to double check names and affiliation when they send galley proof because after publication it's difficult to change them , but the best approach is contacting editor in chief for explaining the mistake , maybe they fix it