Para trocar os repositórios do Ubuntu 12.x favor entrar no SSH como root, digitar os seguintes comandos:

--- Fazendo Backup do Repositorio Antigo
cp /etc/apt/sources.list /etc/apt/sources.list.backup

-- Limpando repositório.
rm /etc/apt/sources.list   

-- Editando Novo Repositorio
-- nano /etc/apt/sources.list

Copie tudo que a seguir:
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://mirror.symnds.com/ubuntu/ precise main restricted
deb-src http://mirror.symnds.com/ubuntu/ precise main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://mirror.symnds.com/ubuntu/ precise-updates main restricted
deb-src http://mirror.symnds.com/ubuntu/ precise-updates main restricted

## 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://mirror.symnds.com/ubuntu/ precise universe
deb-src http://mirror.symnds.com/ubuntu/ precise universe
deb http://mirror.symnds.com/ubuntu/ precise-updates universe
deb-src http://mirror.symnds.com/ubuntu/ precise-updates 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://mirror.symnds.com/ubuntu/ precise multiverse
deb-src http://mirror.symnds.com/ubuntu/ precise multiverse
deb http://mirror.symnds.com/ubuntu/ precise-updates multiverse
deb-src http://mirror.symnds.com/ubuntu/ precise-updates 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.
deb http://mirror.symnds.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://mirror.symnds.com/ubuntu/ precise-backports main restricted universe multiverse

deb http://mirror.symnds.com/ubuntu/ precise-security main restricted
deb-src http://mirror.symnds.com/ubuntu/ precise-security main restricted
deb http://mirror.symnds.com/ubuntu/ precise-security universe
deb-src http://mirror.symnds.com/ubuntu/ precise-security universe
deb http://mirror.symnds.com/ubuntu/ precise-security multiverse
deb-src http://mirror.symnds.com/ubuntu/ precise-security multiverse

## 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 precise partner
deb-src http://archive.canonical.com/ubuntu precise 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 precise main
deb-src http://extras.ubuntu.com/ubuntu precise main

Salve utilizando ctrl + x e aperte y + enter

DIgite: apt-get update


Caso não funcione, volte o backup antigo:

-- Limpando repositório.
rm /etc/apt/sources.list  

--- Retornando Backup do Repositorio Antigo
cp /etc/apt/sources.list.backup /etc/apt/sources.list

Equipe Tier1
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