Updating the arXiv version of paper after it is published in ACM proceedings Before submitting my paper to one of ACM's conferences, I uploaded the earlier version of the paper on arXiv. The paper has now been accepted and published in ACM proceedings. I plan to update the arXiv version of the paper with the final version of the paper that has been published in ACM proceedings. I wanted to ask if: 
1) Am I allowed to do this?
2) Do I have to make any changes to the format of the draft. i.e the paper contains a copyright block from ACM now. 

When granting licence to ACM, I did NOT opt for open-access. Also the paper was published in 2018.
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.