This gets a little tricky. I'm assuming you have some version of Knoppix installed to hard drive (obviously, if you are running from CD you just use a newer version on CD).
The simple answer is that Knoppix installed to hardrive is basically a variant of a Debian system so you would upgrade just as you would with a Debian system using apt-get or aptitude. The caveat is that Knoppix is sufficiently modified from standard Debian that upgrading is not always successful, sometimes creating enough problems so that a reinstall is required.
If you want to try, the best advice is to upgrade individual packages only, not the whole system...ie do not try 'apt-get upgrade'. The basic procedure is first 'apt-get update' to update your package lists then 'apt-get install <packagename>' to upgrade the package. You can specify 'apt-get -t unstable install <packagename>' to get the very latest release of the package if you want to.
Read up on using apt-get before you try it & post back if you get problems.
Clean up your downloaded replacement packages afterward with 'apt-get clean', you could end up with several hundred meg of accumulated package files wasting drive space otherwise.