首先编辑软件源配置文件sources.list
nano命令
nano 文件名：打开或新建文件
保存: ^O = Ctrl + O
离开 : ^X = Ctrl + X
sudo nano /etc/apt/sources.list
用#注释掉原先的内容，然后添加清华 TUNA 的软件源镜像
根据ubuntu版本选择对应镜像源，获取ubuntu版本：
yeqiuyi@yeqiuyi-hp:~$ cat /etc/issue```
ubuntu 18.04 LTS
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
然后Ctrl+X，Y，确认
更新软件源：
sudo apt-get update
yeqiuyi@yeqiuyi-hp:~$ sudo apt-get update
命中:1 http://mirrors.shu.edu.cn/ubuntu bionic InRelease
获取:2 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic InRelease [242 kB]
获取:3 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates InRelease [88.7 kB]
获取:4 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports InRelease [74.6 kB]
获取:5 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security InRelease [83.2 kB]
......
获取:71 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security/multiverse i386 Packages [1,608 B]
获取:72 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security/multiverse Translation-en [996 B]
已下载 45.4 MB，耗时 7秒 (6,865 kB/s)
正在读取软件包列表... 完成
此步骤之前我还把sources.list.d文件夹内的文件(PPA)删除了，不知道有没有用
sudo rm 文件名