Managing installed software (so called packets) is pretty straightforward on linux systems like Debian.
Go to the console and edit the /etc/apt/sources.list Sometimes the location is a bit different, like /etc/sources.list
You can install software automatically from repositories, accessible via the internet. Adding a line like
deb ftp://ftp.de.debian.org/debian/ unstable main
to sources.list adds such a repository.
you can install software with
apt-get install xxxxx
update the packets list with
and after this, you can upgrade all installed packets to the latest version available with