Imagen de Gabriel_M
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...... abre una terminal, edita tu archivo sources.list

sudo su
nano /etc/apt/sources.list

Pega este contenido

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu quantal main restricted
deb-src http://archive.ubuntu.com/ubuntu quantal restricted main multiverse universe #Added by software-properties

## Major bug fix updates produced after the final release of the
## distribution.

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu quantal universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://archive.ubuntu.com/ubuntu quantal multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.


## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu quantal partner
deb-src http://archive.canonical.com/ubuntu quantal partner

## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu quantal main
deb-src http://extras.ubuntu.com/ubuntu quantal main
deb http://security.ubuntu.com/ubuntu/ quantal-security restricted main multiverse universe
deb http://archive.ubuntu.com/ubuntu quantal-updates restricted main multiverse universe
deb http://archive.ubuntu.com/ubuntu quantal-proposed restricted main multiverse universe
deb http://archive.ubuntu.com/ubuntu quantal-backports restricted main multiverse universe

Guarda el archivo (Control + O)
Cierra nano (Control + X)
y en la misma terminal continua ejecutando

apt-get update
apt-get dist-upgrade
dpkg --configure -a
apt-get clean
apt-get autoremove
reboot
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