A pull request is just that: a request to merge one branch into another.
Your pull request doesn't "contain" anything, it's just a marker saying "please merge this branch into that one".
The set of changes the PR shows in the web UI is just the changes between the target branch and your feature branch. To modify your pull request, you must modify your feature branch, probably with a force push to the feature branch.
In your case, you'll probably want to amend your commit. Not sure about your exact situation, but some combination of interactive rebase and add -p
should sort you out.