There are three ways one can do that: from the web UI (Option 1), from the GitHub CLI (Option 2), or from the command line (Option 3).

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Option 1 – Web UI

1. On GitHub, navigate to the main page of the forked repository that you want to sync with the upstream repository.

2. Select the Fetch upstream drop-down.

3. Review the details about the commits from the upstream repository, then click Fetch and merge.

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Option 2 – GitHub CLI

To update the remote fork from its parent, use the gh repo sync subcommand and supply your fork name as argument.

$ gh repo sync owner/cli-fork

If the changes from the upstream repository cause conflict then the GitHub CLI can’t sync. You can set the -force flag to overwrite the destination branch.

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Option 3 – Command Line

Before syncing one’s fork with an upstream repository, one must configure a remote that points to the upstream repository in Git.

1 Open Git Bash.

2 Change the current working directory to your local project.

3 Fetch the branches and their respective commits from the upstream repository. Commits to BRANCHNAME will be stored in the local branch upstream/BRANCHNAME.

$ git fetch upstream
> remote: Counting objects: 75, done.
> remote: Compressing objects: 100% (53/53), done.
> remote: Total 62 (delta 27), reused 44 (delta 9)
> Unpacking objects: 100% (62/62), done.
> From https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY
>  * [new branch]      main     -> upstream/main

4 Check out your fork’s local default branch – in this case, we use main.

$ git checkout main
> Switched to branch 'main'

5 Merge the changes from the upstream default branch – in this case, upstream/main – into your local default branch. This brings your fork’s default branch into sync with the upstream repository, without losing your local changes.

$ git merge upstream/main
> Updating a422352..5fdff0f
> Fast-forward
>  README                    |    9 -------
>  README.md                 |    7 ++++++
>  2 files changed, 7 insertions(+), 9 deletions(-)
>  delete mode 100644 README
>  create mode 100644 README.md

If one’s local branch didn’t have any unique commits, Git will instead perform a “fast-forward”:

$ git merge upstream/main
> Updating 34e91da..16c56ad
> Fast-forward
>  README.md                 |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)

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Note: Syncing one’s fork only updates one’s local copy of the repo.
To update one’s fork on GitHub.com, one must push ones changes.

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Source: GitHub Docs – Syncing a fork