Step 1: modify lsb-release file

ubuntu@ubuntu:~$ sudo nano /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty
DISTRIB_DESCRIPTION="Ubuntu 14.04 LTS"

Save file and exit .

Step 2: modify sources.list

ubuntu@ubuntu:~$ sudo nano /etc/apt/sources.list
deb http://ports.ubuntu.com/ubuntu-ports/ trusty main universe
deb-src http://ports.ubuntu.com/ubuntu-ports/ trusty main universe
deb http://ports.ubuntu.com/ubuntu-ports/ trusty-security main universe
deb-src http://ports.ubuntu.com/ubuntu-ports/ trusty-security main universe
deb http://ports.ubuntu.com/ubuntu-ports/ trusty-updates main universe
deb-src http://ports.ubuntu.com/ubuntu-ports/ trusty-updates main universe
#deb http://www.wiimu.com:8020/pcduino/ pcduino main
#deb-src http://www.wiimu.com:8020/pcduino/ pcduino main

Save file and exit .

Step 3: update sources.list

$ sudo apt-apt update

Step 4: upgrade

$ sudo apt-apt upgrade