大家好，又见面了，我是你们的朋友全栈君。

~# cat /etc/issue
Ubuntu 18.04.5 LTS \n \l

# apt update
Ign:1 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic InRelease
Ign:2 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates InRelease
Ign:3 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports InRelease
Ign:4 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security InRelease
Err:5 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic Release
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Err:6 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates Release
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Err:7 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports Release
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Err:8 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security Release
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Reading package lists... Done
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates Release' no longer has a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security Release' no longer has a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

https 替换为 http

# cat /etc/apt/sources.list
# 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 http://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 http://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 http://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 http://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

# apt update
Hit:1 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic InRelease
Get:2 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates InRelease [88.7 kB]
Get:3 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports InRelease [74.6 kB]
Get:4 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security InRelease [88.7 kB]
Get:5 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates/main amd64 Packages [2,303 kB]
Get:6 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates/main Translation-en [445 kB]
Get:7 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates/restricted amd64 Packages [540 kB]
Get:8 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates/restricted Translation-en [73.6 kB]
Get:9 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates/universe amd64 Packages [1,764 kB]
Get:10 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates/universe Translation-en [380 kB]
Get:11 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates/multiverse amd64 Packages [27.3 kB]
Get:12 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates/multiverse Translation-en [6,808 B]
Get:13 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security/main amd64 Packages [1,958 kB]
Get:14 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security/main Translation-en [352 kB]
Get:15 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security/restricted amd64 Packages [516 kB]
Get:16 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security/restricted Translation-en [69.9 kB]
Get:17 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security/universe amd64 Packages [1,148 kB]
Get:18 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security/universe Translation-en [263 kB]
Get:19 http://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security/multiverse amd64 Packages [20.9 kB]
Fetched 10.1 MB in 7s (1,372 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
177 packages can be upgraded. Run 'apt list --upgradable' to see them.