After installing Ubuntu server, the very first thing we need to do is to replace the mirror sources to domestic ones (such as aliyun). However, in the command interface, it is a tedious work to type numbers of long URLs into the sources.list, so I copy the source file from my USB disk. And if you have a GUI, it's more efficient to copy the URLs from this page.

• Check the codename

Use the command

Ubuntu~$ lsb_release -c

to check the codename of your system
and the interface will show as:

Codename:      xenial

The xenial is a codename for Ubuntu 16.04, and may vary with different systems. And for you, it may be bionic or others.
Remeber it, which will be used next.

• Create a sources.list in a GUI

Add the following content to the sources.list:

deb http://mirrors.aliyun.com/ubuntu/ codename main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ codename main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ codename-security main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ codename-security main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ codename-updates main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ codename-updates main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ codename-backports main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ codename-backports main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ codename-proposed main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ codename-proposed main restricted universe multiverse

Remember to replace the string codename to your real codename.

• Put your sources.list to the USB disk

Make sure that your USB device is NTFS or FAT32 file system, for that the Ubuntu don't support a exFAT system by default.

• Mount the USB disk to your Ubuntu

See details in: USB Disk Mounting in the Ubuntu Server and you will mount the U-disk to /mnt/usb

• Bakeup your legacy sources.list

Use the command:

Ubuntu~$ sudo mv /etc/apt/sources.list /etc/apt/sources.list.back
Ubuntu~$ sudo cp /mnt/usb/sources.list /etc/apt/sources.list

to rename your legacy sources and put the new list to the dirtionary.

• Update the mirror sources

Use the command:

Ubuntu~$ sudo apt update
Ubuntu~$ sudo apt upgrade

to update your apt sources and upgrade software packages.