Add that to your sources.list:
deb http://backports.debian.org/debian-backports squeeze-backports main
If you’re interested in just upgrading the usual mesa packages:
apt-get install -t squeeze-backports libgl1-mesa-dri libgl1-mesa-glx
If you want the whole stack, we should distinguish between two
cases.
In the usual case, both the xserver-xorg-input-all and
xserver-xorg-video-all meta packages were installed, and pulled a
lot of packages, which should work for most users.
Specifying a few packages to upgrade should pull
everything from squeeze-backports.
apt-get install -t squeeze-backports xorg xserver-xorg xserver-xorg-core xserver-xorg-input-all xserver-xorg-video-all
apt-get install -t squeeze-backports libgl1-mesa-dri libgl1-mesa-glx
In case only the needed packages were installed, like the synaptics
input driver and the intel video driver, the following should be
sufficient:
apt-get install -t squeeze-backports xorg xserver-xorg xserver-xorg-core xserver-xorg-input-synaptics xserver-xorg-video-intel
apt-get install -t squeeze-backports libgl1-mesa-dri libgl1-mesa-glx
It is probably a very good idea to install the Linux kernel from
squeeze-backports as well. It is even required for the nouveau
video driver.