1. List the current configured remote repository for your fork.
    git remote -v
    # origin  https://github.com/YOUR_USERNAME/YOUR_FORK.git (fetch)
    # origin  https://github.com/YOUR_USERNAME/YOUR_FORK.git (push)
    
  2. 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
    
  3. Verify the new upstream repository you've specified for your fork.
    git remote -v
    # origin    https://github.com/YOUR_USERNAME/YOUR_FORK.git (fetch)
    # origin    https://github.com/YOUR_USERNAME/YOUR_FORK.git (push)
    # upstream  https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git (fetch)
    # upstream  https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git (push)