Security updates for your version of Debian (lenny) have been discontinued as of February 6th, 2012.
Source: http://www.debian.org/releases/lenny/
So you will also need to update to Debian 6 (squeeze) at least.
To upgrade:
- backup all your data and configuration
Update package repositories:
deb http://ftp.se.debian.org/debian/ lenny main contrib non-free deb-src http://ftp.se.debian.org/debian/ lenny main contrib non-free deb http://security.debian.org/ lenny/updates main deb-src http://security.debian.org/ lenny/updates main deb http://volatile.debian.org/debian-volatile lenny/volatile main deb-src http://volatile.debian.org/debian-volatile lenny/volatile main
Fetch new repositories for Lenny
apt-get update
First upgrade all your packages:
apt-get upgrade
Then do dist-upgrade:
apt-get dist-upgrade
More info: http://www.howtoforge.com/upgrade-debian-lenny-to-squeeze-in-a-few-simple-steps