Ubuntu 20.04 LTS Focal Fossa于2020年04月23日发布。

备份
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak

更改文件权限使其可编辑
sudo chmod 777 /etc/apt/source.list

写入
gedit /etc/apt/sources.list

#阿里源
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse

#清华源
# 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

#网易源
deb http://mirrors.163.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ focal-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ focal-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ focal-backports main restricted universe multiverse

#中科大源
deb https://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse

终端逐个运行下列代码
apt-get update
apt-get upgrade
就可以了

拓展
Ubuntu 官方（歐洲，國內較慢，無同步延遲）http://archive.ubuntu.com/ubuntu/
Ubuntu 官方中國（目前是阿里云）http://cn.archive.ubuntu.com/ubuntu/
網易（廣東廣州電信/聯通千兆雙線接入）http://mirrors.163.com/ubuntu/
搜狐（山東聯通千兆接入）http://mirrors.sohu.com/ubuntu/
阿里云（北京萬網/浙江杭州阿里云服務器雙線接入）http://mirrors.aliyun.com/ubuntu/
详情见：http://wiki.ubuntu.org.cn/模板:20.04source

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