In your local clone of your forked repository, you can add the original GitHub repository as a "remote". ("Remotes" are like nicknames for the URLs of repositories - origin is one, for example.) Then you can fetch all the branches from that upstream repository, and rebase your work to continue working on the upstream version. Taken form: http://…

Update a forked project

Step 1: 
# Add the remote, call it "upstream":
$ git remote add upstream git://github.com/whoever/whatever.git

Step 2: 
# Fetch all the branches of that remote into remote-tracking branches, such as upstream/master:
$ git fetch upstream

Step 3: 
# Make sure that you're on your master branch:
$ git checkout master

Step 4: 
# 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

Step 5:
# If you've rebased your branch onto upstream/master you may need to force the push 
# in order to push it to your own forked repository on GitHub. You'd do that with:
$ git push -f origin master

You only need to use the -f the first time after you've rebased.