GitHub–Steps to merge a fork to its parent

3 simple steps: (this assumes you have setup a remote branch called upstream that points at the original repo)

1. git fetch upstream
2. git checkout master
3. git merge upstream/master

And to setup a remote called upstream that points at the original repo:

git remote add upstream {url to the original repo}

for more info see: https://help.github.com/articles/syncing-a-fork/