Do NOT install this PPA if you want a tested running system, this PPA contains the newest and most unstable development of elementary, it’s useful only if you are a developer and are not afraid to encounter CRITICAL BUGS.
Also this PPA may overwrite already installed packages you don’t want it to!
sudo add-apt-repository ppa:elementary-os/daily
sudo apt-get update
#if cannot add ppa, try add to sources.list(/etc/apt/)