Knoppix is based on Debian Woody testing. You will find more software packages on any Debian mirror. Please look at

http://www.debian.org/

For updating and downloading software you should be familiar with apt-get, a piece of software that does all the downloading and installing for you. Please read the documentations at www.debian.org. There are a number of very good introductions to apt-get in the net, just try to locate them via Google.