I realize this is an old question, but GitHub has recently introduced some new features that make it possible to actually update a pull request submitted by another user.
When you are creating a new pull request, you'll see a checkbox labelled "Allow edits from maintainers". This is enabled by default.
With this in place, anyone with commit access to the repository that is the target of your pull request will also be able to push changes to the branch of your repository that is the origin of the pull request.
This is especially use in team environments in which everyone has commit access to the "main" repository, but all work is done on feature branches in individual forks. It means that if there is an open pull request that needs some changes and the primary author is unavailable, someone else on the team can make the necessary updates directly, rather than closing the existing PR and opening a new one.