Please see the file /etc/apt/sources.list

Peruse the Debian method of marking [oldoldstable] [oldstable] [stable] [testing] [unstable] [experimental] and various other kinds.

Then find your installed Knoppix version's (not the Debian Current Version) declaration in the sources file at the very top.

Re-arrange your declarations accordingly. It is simple and common-sense.

Also don't forget to install the security keys by hand.

Can't be done unless you are Superuser.

That is all there is.