The Kubuntu team has pushed an update of KDE 4.9.1 Plasma Workspaces, Applications and Platform in their repositories. This is the first distro to adopt KDE 4.9.1. KDE 4.9.1 comes as a bug fix release over the previous KDE 4.9 series desktop.
To add this repository, you need to add a PPA. Run the following in terminal:
sudo apt-add-repository ppa:kubuntu-ppa/backports
sudo apt-get update
sudo apt-get upgrade