Some applications require a newer kernel version to make them work better . But the current stable kernel is 2.6.32.
First of all;
1. backup your /boot!
cp /boot /root -R
2. Make changes to your sources.list .
3. Add the line below to the bottom of the file.
deb http://backports.debian.org/debian-backports squeeze-backports main
4. Update your repositories
5. Install the backport and the kernel. (note: the linux headers and other dependencies will be installed aswell)
apt-get install -t squeeze-backports linux-image-2.6.39-bpo.2-amd64
The install will update the grub config aswell, and will put the 2.6.39 kernel at the top (so it will boot first). If all goes well you should reboot the server and the kernel will be loaded.
If the server doesnt reboot and gives you the grub shell . You can try to boot from a live disk, and copy the files from your backup back to /boot of your install.