The Debian developers announced last month that an outside team would be providing Long Term Support (LTS) for Debian 6.0 aka Squeeze. This means that you can continue using Debian Squeeze until February 2016.
To enable Debian Squeeze LTS updates you have to add the LTS repository to /etc/apt/sources.list. This is what you add:
deb http://http.debian.net/debian/ squeeze-lts main
Then update to download the latest security fixes:
apt-get update && apt-get upgrade