Conclusion

All you have to do is rename the branch in GitHub.

1. Open your repository's branches page (/{your_user_id}/{your_repository_name}/branches) and rename the branch.

2. Then, all PRs that had the branch as a base branch automatically are updated and the base branch is also changed to the renamed one.

※ But, the PR that had the branch as a head branch doesn't be updated automatically. So, you need to re-create a new pull request once more.

cf. https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch