To edit a pull request, select the pull request on the Pull requests page, go to its detail page and click "Edit."
Once a pull request is created, the target branch (the base branch) and the featured branch for the pull request cannot be changed.
If you need to change the target/feature branches, simply close the current pull request and create a new one. In this case, remember to close only the pull request itself, rather than deleting the feature branch of code that is associated with the pull request.