Changing source for packages: lines in sources.list apt.conf.d 00knoppix

[rajibando]

I wish to change the default source for Synaptic. I changed the line in:

/etc/apt/apt.conf.d/00knoppix

From:

APT::Default-Release "wheezy";

To:

APT::Default-Release "squeeze-backports";

and

/etc/apt/sources.list

From:

deb http://ftp.de.debian.org/debian stable main contrib non-free

and other lines
To:

deb http://ftp.de.debian.org/debian-backports/ squeeze-backports main non-free

The original files have been renamed and kept, and can be restored.
Synaptic gives this error message

E: The value 'squeeze-backports' is invalid for APT::Default-Release as such a release is not available in the sources
E: _cache->open() failed, please report.

and closes.
What more needs to be done?
The change is required to clean reinstall nvidia drivers and repair the following error codes as Output:
Synaptic_Changes_Applied_terminal.txt
For nvidia drivers the following errors are known to the system:

ERROR: The conflicting library '/usr/lib/i386-linux-gnu/libGL.so.1.2.0' is known to dpkg.