If you have a paid account, you can convert your public repositories to private ones.

Tip: If you are on the free plan, or have no private repositories available, you will need to upgrade before you can switch the repository to private.

  1. Open the repository's page on GitHub.
  2. In your repository's right sidebar, click . Repository settings button

  3. Click Make private. Make Private button

  4. Read the warnings. Warnings popup
  5. Type the name of the repository that you want to make private.
  6. Click I understand, make this repository private. Warning Popup

Warning: When a public repository is made private, public forks of the public repository are detached and put into a new network. They are not made private. For more information, see "What happens to forks when a repository is deleted or changes visibility".