List the remote for your fork
$ git remote -v
Add a new remote "upstream" that will be synced with the fork
$ git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git
Verify the new upstream repository
$ git remote -v
Go to your local project and fetch the remote, bringing the branches and their commits from the upstream repository. You can use the -p, --prune option to delete any remote-tracking references that no longer exist in the remote. Commits to master will be stored in a local branch, upstream/master
$ git fetch upstream
Check out the branch you want to merge into (your fork's local master branch)
$ git checkout master
Merge the changes from upstream/master into your local master branch. This brings your fork's master branch in sync with the upstream repository, without losing your local changes. If your local branch didn't have any unique commits, Git will instead perform a "fast-forward".
$ git merge upstream/master
Syncing your fork only updates your local repository. To update your fork on GitHub, push your changes.
$ git push origin master