Find word squeeze and replace with wheezy. At the end it should look as follows:

 
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

# wheezy-updates, previously known as 'volatile'
deb http://mirrors.kernel.org/debian/ wheezy-updates main
deb-src http://mirrors.kernel.org/debian/ wheezy-updates main

To update the package list, enter:

# apt-get update

do a update of the base system, enter:

# apt-get upgrade

Now upgrade the rest of the system, enter:

# apt-get dist-upgrade

Reboot the system:

you can verify your debian version by entering:

$ lsb_release -a

the output should look something like this:

No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 7.0 (wheezy)
Release:	7.0
Codename:	wheezy