Updating the arXiv version of paper after it is published in ACM proceedings
According to the ACM Copyright Transfer Agreement, you are allowed to


  Post the Accepted Version of the  Work on (1) the Author’s home page,  (2) the Owner’s institutional repository, or (3) any repository legally mandated by an agency funding the research on which the Work is based, and (4) any non-commercial repository or aggregation that does not duplicate ACM tables of contents, i.e., whose  patterns of links do not substantially duplicate an ACM-copyrighted volume  or issue. Non-commercial repositories  are here understood as repositories  owned by nonprofit organizations that  do not charge a fee for accessing deposited articles and that do not sell advertising or otherwise profit from serving articles. 


So, yes, you can update your manuscript on arXiv with the accepted version but you should not use the published version (version of record), i.e. the one with the ACM copyright. Remove the ACM style by, e.g. using another style sheet in LaTeX.