第一步
在清华大学开源镜像站找到自己的linux系统版本
我的是Ubuntu 18.04LTS:
# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
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
# 预发布软件源，不建议启用
# 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
第二步
apt-get配置文件所在的目录为/etc/apt，故先进入目录，然后对原配置文件进行一份备份：sudo cp sources.list sources.list.bak，以防丢失。最后使用vim打开sources.list文件，将原内容删除，替换为新的配置内容（在root模式下进行）。备份命令如下：
ubuntu:~$ cd /etc/apt
ubuntu:/etc/apt$ sudo cp sources.list sources.list.bak
第三步
更新源，使配置生效
ubuntu:~$ sudo apt-get update