To sync changes you make in a fork with the original repository, you must configure a remote that points to the upstream repository in Git.
Specify a new remote upstream repository that will be synced with the fork.
git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git
You can check if it was succesful with:
git remote -v
Then fetch it to update your project:
git fetch upstream
Merge the changes from upstream/master into your local master branch.
git merge upstream/master
At last, you can commit new update from original repository to your fork repository.
Shortcut: you can also combine the last two commands into a single command:
git fetch upstream
git merge upstream/master
Is equal to:
git pull upstream/master
This information can also be found on GitHub here and here.