Syncing fork with upstream

sync_fork.sh

# Foreword: Your fork is the "origin" and the repository you forked from is the "upstream".
# Let's assume that you cloned already your fork to your computer with a command like this:
#
git clone git@github.com:your_name/project_name.git
cd project_name

# If that is given then you need to continue in this order:
#
# 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!

GitHub also provides instructions for this topic:
Syncing a fork (https://help.github.com/articles/syncing-a-fork)