on the second line, you see the antix/daveserver repo, and it is set to stable.
If you want to change that to testing, do this:
enter your password
in that file, you will see several lines, most are commented out (has a # at the start of the line)
You need to add a # on the line that has stable in it, and remove the # on the line who says testing

After that, apt-get update again, then try to install the kernel

Wenn Du Dich mit NANO auskennst, kannst Du das so machen?