AFAIK, you cannot change the source branch after creating a Pull Request. You have to create a new one instead.
For future reference, the established best practice is to create a new branch before making any commits. You should not commit directly to master, especially when contributing to team projects.
Side note:
you can simply Add more commits by pushing to the master branch on username/repo
More correctly, any changes to the target branch of a PR are automatically included in the PR.