Board Thread:Support Requests - Getting Started/@comment-36387140-20180824173951/@comment-1038387-20180824180455

The rename is handled by the UserRenameTool. It's moved. Done. Your old account name is now a redirect to the new name, but the contributions are credited to the new name.

You can always try to ask staff at Special:Contact/general, but I don't think it can be undone. That it was 10 minutes ago doesn't matter.