ubuntu14.04更换源

1、备份

sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
sudo vim /etc/apt/sources.list

2、删除/etc/apt/sources.list文件中的全部内容
增加如下内容

deb http://mirrors.163.com/ubuntu/ precise main restricted
deb-src http://mirrors.163.com/ubuntu/ precise main restricted
deb http://mirrors.163.com/ubuntu/ precise-updates main restricted
deb-src http://mirrors.163.com/ubuntu/ precise-updates main restricted
deb http://mirrors.163.com/ubuntu/ precise universe
deb-src http://mirrors.163.com/ubuntu/ precise universe
deb http://mirrors.163.com/ubuntu/ precise-updates universe
deb-src http://mirrors.163.com/ubuntu/ precise-updates universe
deb http://mirrors.163.com/ubuntu/ precise multiverse
deb-src http://mirrors.163.com/ubuntu/ precise multiverse
deb http://mirrors.163.com/ubuntu/ precise-updates multiverse
deb-src http://mirrors.163.com/ubuntu/ precise-updates multiverse
deb http://mirrors.163.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ precise-backports main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ precise-security main restricted
deb-src http://mirrors.163.com/ubuntu/ precise-security main restricted
deb http://mirrors.163.com/ubuntu/ precise-security universe
deb-src http://mirrors.163.com/ubuntu/ precise-security universe
deb http://mirrors.163.com/ubuntu/ precise-security multiverse
deb-src http://mirrors.163.com/ubuntu/ precise-security multiverse
deb http://extras.ubuntu.com/ubuntu precise main
deb-src http://extras.ubuntu.com/ubuntu precise main

3、更新

sudo apt-get update

报错

The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192

解决:这个非官方源是不可信任的，解决办法是导入该源的公钥

gpg --keyserver subkeys.pgp.net --recv 16126D3A3E5C1192
gpg --export --armor 16126D3A3E5C1192 | sudo apt-key add -
sudo apt-get update
sudo apt-get upgrade

#deb cdrom:[Ubuntu 14.10 _Utopic Unicorn_ - Release amd64 (20141022.1)]/ utopic main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://cn.archive.ubuntu.com/ubuntu/ utopic main restricted
deb-src http://cn.archive.ubuntu.com/ubuntu/ utopic main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://cn.archive.ubuntu.com/ubuntu/ utopic-updates main restricted
deb-src http://cn.archive.ubuntu.com/ubuntu/ utopic-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://cn.archive.ubuntu.com/ubuntu/ utopic universe
deb-src http://cn.archive.ubuntu.com/ubuntu/ utopic universe
deb http://cn.archive.ubuntu.com/ubuntu/ utopic-updates universe
deb-src http://cn.archive.ubuntu.com/ubuntu/ utopic-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://cn.archive.ubuntu.com/ubuntu/ utopic multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ utopic multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ utopic-updates multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ utopic-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://cn.archive.ubuntu.com/ubuntu/ utopic-backports main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ utopic-backports main restricted universe multiverse

deb http://security.ubuntu.com/ubuntu utopic-security main restricted
deb-src http://security.ubuntu.com/ubuntu utopic-security main restricted
deb http://security.ubuntu.com/ubuntu utopic-security universe
deb-src http://security.ubuntu.com/ubuntu utopic-security universe
deb http://security.ubuntu.com/ubuntu utopic-security multiverse
deb-src http://security.ubuntu.com/ubuntu utopic-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 utopic partner
# deb-src http://archive.canonical.com/ubuntu utopic 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 utopic main
deb-src http://extras.ubuntu.com/ubuntu utopic main