You would want to amend the commit and then do a force push which will update the branch with the PR.
Here's how I recommend you do this:
git reset --soft HEAD^
or if it's a different commit, you would want to replace 'HEAD^' with the commit id)git commit -a -c ORIG_HEAD
The now that your branch has been updated, the Pull Request will include your changes.
Here's a link to Gits documentation where they have a pretty good example under Undo a commit and redo.