The way to update the software in Ubuntu is as follows three steps:
1. Back up the original software source files
To set the source file in /etc/apt/sources.list , first back up the file Cp/etc/apt/sources.list/etc/apt/sources.list.bak
2. Add the appropriate source
Generally I am the source of the added NetEase
Deb Http://mirrors.163.com/ubuntu/trusty main restricted universe Multiverse
Deb http://mirrors.163.com/ubuntu/ Trusty-security main restricted universe Multiverse
Deb Http://mirrors.163.com/ubuntu/trusty-updates main Restricted universe Multiverse
Deb http://mirrors.163.com/ubuntu/trusty-proposed main restricted universe Multiverse
Deb Http://mirrors.163.com/ubuntu/trusty-backports main restricted universe multiverse
DEB-SRC Http://mirrors.163.com/ubuntu/trusty main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ Trusty-security main restricted universe Multiverse
deb-src http://mirrors.163.com/ubuntu/trusty-updates Main Restricted universe Multiverse
deb-src http://mirrors.163.com/ubuntu/trusty-proposed main restricted universe Multiverse
deb-src http://mirrors.163.com/ubuntu/trusty-backports main restricted universe multiverse
3. Refresh Source
sudo apt-get update