Updating a paper on arXiv that was accepted by a non-open access journal
[The comments mostly answer this, but posting an answer as well for completeness]

AM - Accepted Manuscript (you may see the term AAM/Author's Accepted Manuscript or "postprint", as well). This is the version of the text at the point the journal accepts it - so with any revisions that were requested by reviewers, but before any final copyediting, formatting, etc.

VOR - Version of Record - the final published version from the journal with pretty formatting, pagination, and so on.

You should add the credit line (effectively a citation to the final version) to the arXiv landing page, but it's always a good idea to put it on the paper as well - perhaps a note on the first page to ensure that the reader has a handy reference for the correct citation.

The credit line should be to the published version (with DOI etc) but if this has not yet appeared, then something like "Accepted for publication in the Journal of Widgets, copyright the International Widget Research Society, posted with permission" will do - you can then update this as and when you have the correct details.