An apt-get upgrade will almost certainly either fail or break a HD install of Knoppix. You could do a lot worse than read the first few lines of /etc/apt/sources.list

gvim /etc/apt/sources.list:

Code:
# /etc/apt/sources.list for Knoppix
# If you want to do a "full upgrade", you should first
# upgrade the Packages from Debian/unstable (KDE & Co.)
# before doing a (dist-)upgrade for Debian/testing.

# See sources.list(5) for more information, especialy
# Remember that you can only use http, ftp or file URIs
# CDROMs are managed through the apt-cdrom tool.