Most of the time, after forking and working on a project other pull requests are being merged which leaves your copy behind the current state of the project.
This post walks you through a simple guide on how to sync your fork.
This is the remote URL to the original project which you want to update your fork from.
git remote add upstream git://github.com/ORIGINAL-DEV-USERNAME/REPO-YOU-FORKED-FROM.git
Now check if upstream has been added
git remote -v
This will list the remote url's active in the local repo
Run the below command to make a pull.
git pull upstream master
This pulls changes from the original repo and merges it into your local copy. And after this is happy coding :)