»» Debian Tutorials - Upgrade Squeeze to Wheezy Part 2 ««
Very simple, just place all instances of "squeeze" to "wheezy" in /etc/apt/sources.list as root then run "sudo apt-get update" followed by "sudo apt-get upgrade", after that do "sudo apt-get dist-upgrade" which will fully upgrade absolutely everything but will take a while depending on the amount of applications you currently have installed.