打開終端（Open Terminal）
1 將系統自帶的該文件做個備份： sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup
2 修改配置：
打開文件：
sudo gedit /etc/apt/sources.list
將下面的內容粘貼到打開文件的最前面，全部粘貼過去，保存，並關閉。
# 默認註釋了源碼鏡像以提高 apt update 速度，如有需要可自行取消註釋
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
# 預發佈軟件源，不建議啓用
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
如果你對上面的地址有疑惑，或者你的Ubuntu系統不是18.04，可以打開下面的地址，選擇你的Ubuntu版本，複製裏面的鏈接到文本中。
https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/
3.更新
更新源: sudo apt-get update
更新軟件: sudo apt-get upgrade