You have to add the original repository (the one you forked) as a remote.

From the GitHub documentation on forking a repository:

Once the clone is complete your repo will have a remote named “origin” that points to your fork on GitHub.
Don’t let the name confuse you, this does not point to the original repo you forked from. To help you keep track of that repo we will add another remote named “upstream”:

$ cd PROJECT_NAME
$ git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git
$ git fetch upstream

# then: (like "git pull" which is fetch + merge)
$ git merge upstream/master master

# or, better, replay your local work on top of the fetched branch
# like a "git pull --rebase"
$ git rebase upstream/master

There’s also a command-line tool (gh) which can facilitate the operations above.

Here’s a visual of how it works:

See also “Are Git forks actually Git clones?”.

Use:

git remote add upstream ORIGINAL_REPOSITORY_URL

This will set your upstream to the repository you forked from.
Then do this:

git fetch upstream      

This will fetch all the branches including master from the original repository.

Merge this data in your local master branch:

git merge upstream/master

Push the changes to your forked repository i.e. to origin:

git push origin master

Voila! You are done with the syncing the original repository.

In addition to VonC’s answer, you could tweak it to your liking even further.

After fetching from the remote branch, you would still have to merge the commits. I would replace

$ git fetch upstream

with

$ git pull upstream master

since git pull is essentially git fetch + git merge.

This video shows how to update a fork directly from GitHub

Steps:

1. Open your fork on GitHub.
2. Click on Pull Requests.
3. Click on New Pull Request. By default, GitHub will compare the original with your fork, and there shouldn’t be anything to compare if you didn’t make any changes.
4. Click on switching the base. Now GitHub will compare your fork with the original, and you should see all the latest changes.
5. Click on Create a pull request for this comparison and assign a predictable name to your pull request (e.g., Update from original).
6. Click on Create pull request.
7. Scroll down and click Merge pull request and finally Confirm merge. If your fork didn’t have any changes, you will be able to merge it automatically.

If you want to do it without cli, you can do it fully on the Github website.

1. Go to your fork repository.
2. Click on New pull request.
3. Make sure to set your fork as the base repository, and the original (upstream) repository as a head repository. Usually, you only want to sync the master branch.
4. Create a new pull request.
5. Select the arrow to the right of the merging button, and make sure choose to rebase instead of merge. Then click the button. This way, it will not produce unnecessary merge commit.
6. Done.

If you’re using the GitHub desktop application, there is a synchronise button on the top right corner. Click on it then Update from <original repo> near top left.

If there are no changes to be synchronised, this will be inactive.

Here are some screenshots to make this easy.

If there is nothing to lose you could also just delete your fork just go to settings… go to danger zone section below and click delete repository. It will ask you to input the repository name and your password after. After that you just fork the original again.

To automatically sync your forked repository with the parent repository, you could use the Pull App on GitHub.

Refer to the Readme for more details.

For advanced setup where you want to preserve your changes done to the forked repository, refer to my answer on a similar question here.