====== Upgrade Debian Wheezy to Wheezy Backports ======

Backports cannot be tested as extensively as Debian stable, and backports are provided on an as-is basis, with risk of incompatibilities with other components in Debian stable. Use with care!

Add backports to your /etc/apt/sources.list

  deb http://http.debian.net/debian wheezy-backports main contrib non-free

According to Debian documentation, it is therefore recommended to select single backported packages that fit your needs, and not use all available backports.
Again, it is ** not** a recommended configuration to use backports by default for all packages: it could break your system and delete all your data.

Are you still sure to do so? To use backported packages by default, add the following lines to /etc/apt/preferences
  Package: *
  Pin: release a=wheezy-backports
  Pin-Priority: 600

Then, as usual
 apt-get update
 apt-get update
 apt-get dist-upgrade