Github

Open your forked repository.

Compare

Click on compare.

Base

Click on the link switching the base.

If the link switching the base isn't here, switch the bases yourself by using the two dropdownlists and set them up so that your fork is on the left and the original is on the right.

Open Pull Request

Click on the button Create a pull request to display the form.

Detail

Type the title and the comment you want for your pull request.

Create Pull Request

Click on the button Create a pull request to open a new pull request.

Merge Pull Request

Scroll to the bottom of your pull request and click on the button Merge pull request.

Confirm Merge

Click on the button Confirm merge to merge the changes on your forked repository.

Done

Done! Your forked repository is up to date!