Update: There’s now an automated solution

See https://stackoverflow.com/a/66470086/151312

Original Post

As of now Github doesn’t have an unfork option, so the only solution is to delete the forked repo.

Warning: The following solution will delete your cloned repo.
If you want unfork without your repo is being deleted try this https://stackoverflow.com/a/41486339/6335029

Step 1: Goto Settings of Repo you want to unfork

Step 2: Click Delete this Repository

Step 3: Enter your name / Repo name

Step 4: click “I understand the consequences, delete this repository” button