This should be easy and even easier in GitHub. Seriously. Is it an odd concept?
I forked a repository to make my own changes or changes I want to perform a PR on. And I would like to get all the changes that undoubtedly will happen on the original repository.
May I have those changes please? Merge them with my outdated fork?
NO. NO SOUP FOR YOU!
Yikes. Rant over, here's how you do it.
https://help.github.com/en/articles/configuring-a-remote-for-a-fork
https://help.github.com/en/articles/syncing-a-fork