Tip: You can follow these steps to delete forks that belong to you as well. Deleting a forked repository doesn't delete the upstream repository.
Navigate to the repository you want to delete.
Select Settings from the repository action bar
Click Delete this repository in the Danger Zone™ area
Read the warnings
Enter the name of the repository you want to delete
Click I understand the consequences, delete this repository
Danger: Deleting a private repository also deletes all of its forks. Deleting a public repository will not. Also, deleting all your private repositories will not downgrade your account to a Free plan. For more information see the article on changing visibility.