Configure Hugo Server Index Document

How do I update hugo servers index document from index.html to index.htm?

You cannot.

This would be a useful feature.

How would it be useful for you?

I have a website that’s been running for 27 years. I’d like to maintain the links.

I have 2,458 pages that don’t have a file extension or trailing slash. Ex: /Andrew

I have 67 pages that have an htm file extension. Ex: /states/illinois.htm

7 of those pages are index.htm.

I followed the custom output formats documentation to render htm files.

My issue now is the hugo server won’t render my index.htm when I try to access a directory.

To clarify, you’re looking for the Hugo equivalent of:
https://httpd.apache.org/docs/2.4/mod/mod_dir.html#directoryindex

Meaning, you’d like to be able define one or more directory index files in order of precedence.

Yes. However, I’m thinking about setting up redirects for the 7 index.htm → index.html files and calling it a day.

Good move.