You can delete any repository or fork if you're either an organization owner or have admin permissions for the repository or fork. Deleting a forked repository does not delete the upstream repository.
Only members with owner privileges for an organization or admin privileges for a repository can delete an organization repository. If Allow members to delete or transfer repositories for this organization has been disabled, only organization owners can delete organization repositories. For more information, see "Repository permission levels for an organization."
Warning: These steps will permanently delete the repository, wiki, issues, and comments. This action cannot be undone.
Please also keep in mind that:
Deleting a private repository will delete all of its forks.
Deleting a public repository will not delete its forks.
On GitHub, navigate to the main page of the repository.
Under your repository name, click Settings.
Repository settings button
Under Danger Zone, click Delete this repository.
Repository deletion button
Read the warnings.
To verify that you're deleting the correct repository, type the name of the repository you want to delete.
Deletion labeling
Click I understand the consequences, delete this repository.
官方教程：