It's currently hard coded in ChangelogGenerator. It shouldn't be too hard to change that code to accept a configuration option.

I have just submitted a PR to allow for a configurable prefix that introduces defaultTitlePrefix (defaulting to ## for backwards compatability).

Despite this, I cannot help but wonder if the idea of a prefix / heading level should be removed entirely, and just have it configured via the title itself - for example, to get level three headings:

changelog:
  repository: testorg/testrepo
  sections:
  - title: "### :star: New Features"
    labels: ["enhancement"]
    sort: "created"
  - title: "### Bugs"
    labels: ["bug"]
    group: "test"
  issues:
    sort: title
    exclude:
      labels: ["hide"]
  contributors:
    title: "### Nice one!"
    exclude:
      names: ["philwebb"]
  external_links:
  - name: "### Release Notes 1"
    location: "url1"
  - name: "Release Notes 2"
    location: "url2"

Closing in favor of PR #59. We can continue the discussion on the PR.