Currently Github does not have a simple, straightforward way to automatically sync a fork with the original repo.

Python

Execute

curl -sL bit.ly/repo-sync | bash

or

bash <(curl -sL bit.ly/repo-sync)

on your terminal in your fork repo directory.

Now you can push your changes.

Note: Only works if the Git origin URL is SSH (git@github.com/owner/repo.git). The name of the original repo will be called upstream.

This source code is under MIT license.