1. Open the repository's page on your GitHub Enterprise instance.
  2. Settings tabClick Settings.
  3. Make Public buttonClick Make public.
  4. Read the warnings.
  5. Type the name of the repository that you want to make public.
  6. Warning PopupClick I understand, make this repository public.

Warning: When a private repository is made public, its private forks are detached.

For more information, see "What happens to forks when a repository is deleted or changes visibility?".