Updated: as Michael notes below, this is now possible:
You can now change the base branch of an open pull request. After you’ve created a pull request, you can modify the base branch so that the changes in the pull request are compared against a different branch. By changing the base branch of your original pull request rather than opening a new one with the correct base branch, you’ll be able to keep valuable work and discussion.
Click the Edit button by the title of the pull request to reveal the base branch selector.
Old answer
You can’t. Just make a new pull request.