Cómo extraer del repositorio original aguas arriba

$ cd PROJECT_NAME
$ git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git
$ git fetch upstream

# then: (like "git pull" which is fetch + merge)
$ git merge upstream/master master

# or, better, replay your local work on top of the fetched branch
# like a "git pull --rebase"
$ git rebase upstream/master
Agrius