To do this, I listed the following lines.
#edit sources.list file
nano /etc/apt/sources.list
#insert the following lines
deb http://ftp.us.debian.org/debian/ wheezy main
deb-src http://ftp.us.debian.org/debian/ wheezy main
deb http://security.debian.org/debian-security wheezy/updates main contrib non-free
deb http://ftp.us.debian.org/debian/ wheezy-updates main
deb-src http://ftp.us.debian.org/debian/ wheezy-updates main
#update your sources
apt-get update
-----output-------
root@debian7:~# apt-get update
Get:1 http://security.debian.org wheezy/updates Release.gpg [836 B]
Get:2 http://security.debian.org wheezy/updates Release [102 kB]
Hit http://http.debian.net wheezy Release.gpg
Hit http://http.debian.net wheezy-updates Release.gpg
Get:3 http://security.debian.org wheezy/updates/main amd64 Packages [92.5 kB]
Hit http://http.debian.net wheezy Release
Hit http://http.debian.net wheezy-updates Release
Get:4 http://security.debian.org wheezy/updates/contrib amd64 Packages [14 B]
Hit http://http.debian.net wheezy/main Sources
Get:5 http://security.debian.org wheezy/updates/non-free amd64 Packages [14 B]
Hit http://http.debian.net wheezy/main amd64 Packages
Get:6 http://security.debian.org wheezy/updates/contrib Translation-en [14 B]
Hit http://http.debian.net wheezy/main Translation-en
Get:7 http://security.debian.org wheezy/updates/main Translation-en [53.3 kB]
Hit http://http.debian.net wheezy-updates/main Sources
Get:8 http://security.debian.org wheezy/updates/non-free Translation-en [14 B]
Hit http://http.debian.net wheezy-updates/main amd64 Packages
Ign http://http.debian.net wheezy-updates/main Translation-en
Fetched 249 kB in 4s (55.4 kB/s)
Reading package lists... Done
-----output-------
