“apt-get update” always failed to fetch

 Try changing to another mirror in "Software & Updates"

default setting in "System Settings" > "Software & Updates" > "Ubuntu Software" > "Download from" is "Server for United States".

change to http://mirrors.aliyun.com/ubuntu/

sudo rm /var/lib/apt/lists/* -vf



sudo apt-get clean

sudo rm -rf /var/lib/apt/lists/*

apt-get clean

apt-get update 

apt-get upgrade

rm:

-r, -R, --recursive

remove directories and their contents recursively i.e. Folders inside them will be removed also.

-v, --verbose

explain what is being done or show what is happening.

-f, --force

 ignore nonexistent files, never prompt