ubuntu@cx1640-1:~$ cat /etc/apt/sources.list
deb http://us.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu zesty main restricted universe multiverse

ubuntu@cx1640-1:~$ sudo apt-get update
Hit:1 http://us.archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 http://ppa.launchpad.net/hardware-certification/public/ubuntu xenial InRelease
Hit:3 http://us.archive.ubuntu.com/ubuntu xenial-updates InRelease      
Hit:4 http://us.archive.ubuntu.com/ubuntu xenial-security InRelease
Get:5 http://us.archive.ubuntu.com/ubuntu zesty InRelease [256 kB]
Get:6 http://us.archive.ubuntu.com/ubuntu zesty/main Sources [919 kB]
Get:7 http://us.archive.ubuntu.com/ubuntu zesty/restricted Sources [5,076 B]
Get:8 http://us.archive.ubuntu.com/ubuntu zesty/universe Sources [8,528 kB]
Get:9 http://us.archive.ubuntu.com/ubuntu zesty/multiverse Sources [190 kB]                                                    
Fetched 9,899 kB in 13s (708 kB/s)                                                                                             
Reading package lists... Done