I've a fresh Debian 6.0 (Squeeze) installation, and have decided to upgrade it to Wheezy (Debian 7.0).
First you need to know that even though in previous versions aptitude has been recommended, in this version apt-get is the recommended command to work with.
You can read (and it is better if you really do it) the full and official instructions here.
But, if your system is not that critical, or if you have somehow a "clean" system. I mean, you have only installed software using the package management tools, you have not manually "tweaked" anything. Then, you can follow this instructions to upgrade from Debian 6.0 (Squeeze) to Debian 7.0 (Wheezy).
First thing you need to do, is backup all important information you may have. I always backup my /home/ folder and the /etc/ folder.
You can use your favorite text editor, to open /etc/apt/sources.list file.
And change squeeze for wheezy, it may look like this after the change:
deb http://mirrors.kernel.org/debian/ wheezy main deb-src http://mirrors.kernel.org/debian/ wheezy main deb http://security.debian.org/ wheezy/updates main deb-src http://security.debian.org/ wheezy/updates main
As I've told you, my system is very clean.
Upgrade the system
Respecting what the official how-to says, we'll do the upgrade in stages.
Update the sources
Upgrade the Kernel
It seems I already have the 2.6 kernel, but I'm going to follow the instructions anyway.
Here is what I've already have installed.
dpkg -l | grep linux-image
The output is:
ii linux-image-2.6-amd64 2.6.32+29 Linux 2.6 for 64-bit PCs (meta-package) ii linux-image-2.6.32-5-amd64 2.6.32-45 Linux 2.6.32 for 64-bit PCs
So, upgrading kernel.
apt-get install linux-image-2.6-amd64
Good thing I did, it actually upgraded and added packages.
And because I'm using grub, and I want to be sure everything is going to work.
apt-get install udev
Once all these steps have completed, I can now upgrade the distribution so:
I'm now running Debian Wheezy (Testing), I only have to wait for it to be the new stable version of Debian in some more days or few weeks from now.
I've run into minor issues, and I had to execute:
apt-get -f install
and then again
After that, everything just worked.