Check out How to update your PR on GitHub by Sarah Maddox. Here is an excerpt:
So, you’ve created a pull request (PR) on GitHub, and you’ve received some review comments. Now you need to update the PR to address the comments. How do you do that? This post shows you how to update the files in an existing PR, using either the command line or the GitHub UI.
When you ask how…
The full article is available here.