1. First backup Ubuntu12.04 source list
sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup (backup the current source list)
2. Modify the update source
vim /etc/apt/sources.list (opens the Ubuntu 12.04 sources list file)
3. Replace the list inside with the following list
SOHU source deb http://mirrors.sohu.com/ubuntu/ precise main restricted deb-src http://mirrors.sohu.com/ubuntu/ precise main restricted deb http://mirrors.sohu.com/ubuntu/ precise-updates main restricted deb-src http://mirrors.sohu.com/ubuntu/ precise-updates main restricted deb http://mirrors.sohu.com/ubuntu/ precise universe deb-src http://mirrors.sohu.com/ubuntu/ precise universe deb http://mirrors.sohu.com/ubuntu/ precise-updates universe deb-src http://mirrors.sohu.com/ubuntu/ precise-updates universe deb http://mirrors.sohu.com/ubuntu/ precise multiverse deb-src http://mirrors.sohu.com/ubuntu/ precise multiverse deb http://mirrors.sohu.com/ubuntu/ precise-updates multiverse deb-src http://mirrors.sohu.com/ubuntu/ precise-updates multiverse deb http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse deb-src http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse deb http://mirrors.sohu.com/ubuntu/ precise-security main restricted deb-src http://mirrors.sohu.com/ubuntu/ precise-security main restricted deb http://mirrors.sohu.com/ubuntu/ precise-security universe deb-src http://mirrors.sohu.com/ubuntu/ precise-security universe deb http://mirrors.sohu.com/ubuntu/ precise-security multiverse deb-src http://mirrors.sohu.com/ubuntu/ precise-security multiverse deb http://extras.ubuntu.com/ubuntu precise main deb-src http://extras.ubuntu.com/ubuntu precise main
163 source 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
4. Update source
sudo apt-get update
5. The following signature could not be verified because there is no public key: NO_PUBKEY 16126D3A3E5C1192
gpg --keyserver pgpkeys.mit.edu --recv-key 16126D3A3E5C1192 gpg -a --export 16126D3A3E5C1192 | sudo apt-key add -