I am unable to add Cinnamon PPA on Ubuntu 12.04 LTS. The add-apt-repository can't access the PPA. Also, when I visit PPA URL on https://launchpad.net/~gwendal-lebihan-dev/+archive/cinnamon-stable, I get 404 error. Is the Cinnamon PPA deleted?
Because the PPA you are trying to add has been deleted. The first question on How do I install the Cinnamon Desktop? has an edit by the maintainer explaining why.
If you are using any Mint application (like your currnet cinnamon), remove that application and PPA from your system.
now install this one for Ubuntu 12.04:
sudo add-apt-repository ppa:tsvetko.tsvetkov/cinnamon sudo apt-get update sudo apt-get install cinnamon
my recommendation is to install mint themes to have a better experience:
sudo apt-get install mint-themes