When you fork GitHub repository, you usually want to have your fork up to date with the original repository. You can update your fork in a few easy steps. Just look at the following example of the Git commands:
Add the remote, call it upstream:
git remote add upstream https://github.com/whoever/whatever.git
Fetch all the branches of that remote into remote-tracking branches, such as upstream/master:
git fetch upstream
Make sure that you’re on your master branch:
git checkout master
Rewrite your master branch so that any commits of yours that aren’t already in upstream/master are replayed on top of that other branch:
git rebase upstream/master
Original post with solution: http://stackoverflow.com/a/7244456