To modify your patch in a pull request, just push more changes to the branch the pull request is based on. Avoid modifying changes that are already part of the pull request branch or force pushing unrelated changes as that will mess up the pull request and make it hard for reviewers to make sense of it. When Skara integrates the pull request, all the changes will be squashed into a single change anyway.
How do I update a pull request with upstream changes?
If your pull request slips too far behind the target branch (e.g. the master branch in the mainline jdk repository), you will need to pull in changes from master and resolve any conflicts before you can integrate. When doing so, it's usually preferred to merge the changes rather than rebasing, especially if the reviews have already started on the pull request. If changes are rebased, the history in the pull request becomes problematic and hard to follow for reviewers. When Skara integrates the pull request, all the changes will be squashed into a single change anyway.
Can Committers for a project push directly to the project repository?