Suppose you have a file that you want to edit in a repo. Similarly to creating a new file, you can also use the UI on GitHub.com to edit a file.
Click on the file you want to edit. In this case, I’ll edit my penultimate FAQ.md file.
On the page where you can view the contents of the file, to the far right side, there’s a pen icon button to Edit this file.
Clicking on Edit this file will put you in the edit UI, similar to the add new file UI.
And just like in adding a new file, you can specify what the commit message should contain and where the commit should go, either current branch (e.g. master in this image) or a new branch.