Deleting a forked GitHub repository is the same as deleting a normal GitHub repository. Please follow the below steps to delete a forked GitHub repository.
Note: Deleting a forked repo does not affect the original repository from where it’s being forked.
Click to your repository for example
yourUsername/yourRepository for example
Then in the main toolbar of GitHub click on Settings
Click on the “Options”.
Scroll to the bottom of the page to the section called Danger Zone and you will find Delete this repository button.
When you click it another pop up will appear here you need to type in your Github username and the name of your repository in this format
gitHubUsername/nameOfTheRepository and click on the button below which says: I understand the consequences, delete the repository.