Assuming you have a central repository, you need to update your forked repo, simply add the source repository as a remote and then use a standard git pull. You can then push those changes to your forked repo.
There are two options to easily update your fork:
Option 1
Merge the upstream repo into your own...
# Add the remote, call it "upstream":
git remote add upstream git://github.com/whoever/whatever.git
# Make sure that you're on your master branch:
git checkout master
# Merge the upstream master branch to your master branch
git pull upstream master
# Now push your changes to your forked repository on github
git push origin master
Option 2:
Alternatively, you could use rebase to update your fork...
# Add the remote, call it "upstream":
git remote add upstream git://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
# Now push your changes to your forked repo on github...
git push origin master
Github has detailed documentation on working with forked repositories: Github: Fork a Repo