Uploading a revised version of published journal article on arXiv
This has happened before. Just update the arXiv version and indicate in the "Comments" field and ideally also inside the file itself that the version is newer than the published one. Examples of such updates:


https://arxiv.org/abs/0801.3306v4 (sadly, the Comments field isn't too clear here)
https://arxiv.org/abs/1605.04832v1 (here, the fact that this is a post-publication update has been made clear in the file itself)
https://arxiv.org/abs/math/0602634v4


The fact that I found/recalled these three links within 10 minutes should suggest that this is not a rare occurrence, although I wish that people would make the status of the preprint as being a more correct version of the publication clearer. Either way, the update is highly encouraged from a reader's perspective :)