There is no single package to upgrade all of kde, at best there is 'meta-packages' like kdebase & kdeaddons (see http://www.kde.org/download/ for a list). Kde releases only the source code, it is up to package maintainers to create packages for each distro, so if you want the very latest kde you will have to compile it from source. I would not recommend this, I have tried it & I would describe it as a PITA at best.
To get kde 3.4.xxx using debian packages, you will have to wait until the packages are available, then kpackage will be able to install them. Kpackage, btw, is just a gui frontend for the apt-get installer system, I find it more convenient to use apt-get directly. First, update your package lists with 'dselect update' command, then just 'apt-get -t unstable install <package>' to get the latest (well almost the latest) package available.
Newer than unstable, there is 'experimental'. It is not normally recommended as the applications may be buggy but it is available. Kde 3.4.2 is now in experimental so 'apt-get -t experimental install kdebase' will get you started.
It may be buggy, I found k3b from experimental would not work (it just locked up) so I downgraded to 'unstable' by 'apt-get -purge remove k3b' then 'apt-get -t unstable install k3b'.
You are going to end up with a very 'mixed' system which can cause some problems, for instance when I installed kde 3.4.2 it upgraded some lib file that koffice required so as part of the install process it removed kword & kspread. So far I have not been able to replace them with kde 3.4.2 compatible versions.