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.
Knoppix 9.1 Live Linux GNU Bootable USB Flash Drive USA
$19.99
$8.50
$6.80
Knoppix Live GNU Linux System 9.1 on Bootable CD / DVD / USB Flash Drive
$9.99
Linux Knoppix 4.0.2 Installation Disc
$39.99
Knoppix Linux Bootable OS v8.6 "Original Live Operating System" 32G USB Stick
$20.30
KNOPPIX 9.1 LINUX INSTALL & LIVE DVD
$9.99
Acer Aspire One 9 inch Netbook ZG5 512MB RAM 8GB SSD HD Knoppix Linux WiFi VGA
$79.99
Knoppix Linux Bootable OS v8.6 "Original Live Operating System" 16G USB Stick
$19.95