You have always been able to either clone a repo and edit it locally, or click the "pencil" icon and edit in the web browser. And now there is a third way, the GitHub Web-based Editor, based off of Visual Studio Code.
This video will walk you through getting started GitHub Web-based Editor.
Find Me:
Website: http://mickeygousset.com
GitHub: https://github.com/mickeygousset
Twitter: / mickey_gousset
Twitch: / mickeygousset