Git commands that enables you to pull changes from the repository that you forked from, aka "upstream".
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