There are several methods to upgrade only security updates on a debian based system, but this one is my favorite.
First i duplicate the sources.list and remove all entries but the lines with the security repositories
sudo cp /etc/apt/sources.list /etc/apt/security.sources.list
Second i ran these easy to remember command
sudo apt-get upgrade -o Dir::Etc::SourceList=/etc/apt/security.sources.list
Thats it. ;)