方法一

Ubuntu可视化界面修改
在设置中的软件和更新中修改红框内容即可，修改后关闭会提示重启服务选择它即可

方法二

修改源文件
位置：/etc/apt/sources.list
首先备份以便出错后还原：sudo cp /etc/apt/sources.list /etc/apt/sources.list_bak
再修改
下面是sources.list文件内容

# deb cdrom:[Ubuntu 20.04 LTS _Focal Fossa_ - Release amd64 (20200423)]/ focal main restricted# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal main restricted## Major bug fix updates produced after the final release of the
## distribution.
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-updates main restricted## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-updates universe## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-updates multiverse## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu focal partner
# deb-src http://archive.canonical.com/ubuntu focal partnerdeb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted
# deb-src http://security.ubuntu.com/ubuntu focal-security main restricted
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security universe
# deb-src http://security.ubuntu.com/ubuntu focal-security universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security multiverse
# deb-src http://security.ubuntu.com/ubuntu focal-security multiverse# This system was installed using small removable media
# (e.g. netinst, live or single CD). The matching "deb cdrom"
# entries were disabled at the end of the installation process.
# For information about how to configure apt package sources,
# see the sources.list(5) manual.

修改完成后保存退出
更新软件包列表：sudo apt-get update
升级软件包：sudo apt-get upgrade

方法三

需用https的时候（使用上面两种可能会报错）
我使用的清华镜像：https://mirrors.tuna.tsinghua.edu.cn/

讲的还是很详细的

我这里贴出来Ubuntu20.04 LTS的/etc/apt/sources.list内容

# 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/ focal main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse# 预发布软件源，不建议启用
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse