Unfortunately the UI does not support altering the index_document at this time. You can create an application via the doctl that will accomplish what you want.

We allow you to specify a custom index_document at the component level. For your example, an app-spec similar to the following would work.

name: sample-html
static_sites:
- environment_slug: html
  github:
    branch: main
    repo: ...
  name: site
  index_document: index.htm

You could then create it via:

doctl app create --spec <path-to-spec>

I’ll create an internal ticket as well to track better supporting this feature in the UI as well.

Thank you for the feedback!