Add the "upstream" to your cloned repository ("origin"):
git remote add upstream git@github.com:original_author/project_name.git
Fetch the commits (and branches) from the "upstream":
git fetch upstream
Switch to the "master" branch of your fork ("origin"):
git checkout master
Stash the changes of your "master" branch:
git stash
Merge the changes from the "master" branch of the "upstream" into your the "master" branch of your "origin":
git merge upstream/master
Resolve merge conflicts if any and commit your merge
git commit -am "Merged from upstream"
Push the changes to your fork
git push
Get back your stashed changes (if any)
git stash pop
You're done! Congratulations!