Cinnamon 1.3 was released almost a week ago, with version 1.3.1 following shortly and for some reason, the PPA we've posted a while back hasn't been updated with either of these versions. A new PPA has been created so Ubuntu users can easily get updates without having to manually download the .deb files.
To add the new PPA and install the latest Cinnamon in Ubuntu 12.04, 12.10, 13.04 or 13.10, copy/paste the following commands in a terminal:
sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-stable sudo apt-get update sudo apt-get install cinnamon
You can remove the previous PPA (ppa:merlwiz79/cinnamon-ppa), but it's not mandatory and leaving it won't affect anything. A note though: some (possibly all) extensions you may have installed from the ppa:merlwiz79/cinnamon-ppa PPA won't work with the latest Cinnamon 1.3.1. But most of them have been converted to applets and are now built-in anyway.
Update: there's also a nightly Cinnamon PPA which you can use to get new features faster (but it's not stable and you may encounter many bugs!). To add the Cinnamon Nightly PPA and install Cinnamon in Ubuntu 13.10, 13.04, 12.10 or 12.04, use the following commands:
sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-nightly sudo apt-get update sudo apt-get install cinnamon