Make sure to backup all your data before doing this, always backup backup backup!

Update your sources.list:

1. vim /etc/apt/sources.list

Add the following sources:

1. deb http://mirrors.kernel.org/debian/ wheezy main
2. deb-src http://mirrors.kernel.org/debian/ wheezy main
4. deb http://security.debian.org/ wheezy/updates main
5. deb-src http://security.debian.org/ wheezy/updates main
7. # wheezy-updates, previously known as ‘volatile’
8. deb http://mirrors.kernel.org/debian/ wheezy-updates main
9. deb-src http://mirrors.kernel.org/debian/ wheezy-updates main

Update the package list:

1. apt-get update

Update the minimal base system:

1. apt-get upgrade

Upgrade the rest of the system:

1. apt-get dist-upgrade

Reboot the system:

1. reboot

Once the server comes back to life verify you are now running Debian Wheezy:

1. cat /etc/issue

Sample Output:

1. Debian GNU/Linux 7 \n \l