为什么要安装国内源？因为默认的软件更新源是国外的，在国内使用下载速度很慢。我们只需要把它默认更新软件的国外网址改成我们国内的镜像网址就行了。

首先将我们的原来的软件源备份：（这样就算改错了也能恢复）

sudo cp -v /etc/apt/sources.list /etc/apt/sources.list.backup

终端执行chmod命令更改文件权限使软件源文件可编辑：

sudo chmod 777 /etc/apt/sources.list

之后通过gedit命令编辑软件源：

sudo gedit /etc/apt/sources.list

然后将里面的国外的网址改为国内的镜像网址就行了。（下面是修改前的样子）

#deb cdrom:[Ubuntu 21.10 _Impish Indri_ - Release amd64 (20211012)]/ impish main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://cn.archive.ubuntu.com/ubuntu/ impish main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://cn.archive.ubuntu.com/ubuntu/ impish-updates main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish-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://cn.archive.ubuntu.com/ubuntu/ impish universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish universe
deb http://cn.archive.ubuntu.com/ubuntu/ impish-updates universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish-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://cn.archive.ubuntu.com/ubuntu/ impish multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ impish-updates multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish-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://cn.archive.ubuntu.com/ubuntu/ impish-backports main restricted universe multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish-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 impish partner
# deb-src http://archive.canonical.com/ubuntu impish partner

deb http://security.ubuntu.com/ubuntu impish-security main restricted
# deb-src http://security.ubuntu.com/ubuntu impish-security main restricted
deb http://security.ubuntu.com/ubuntu impish-security universe
# deb-src http://security.ubuntu.com/ubuntu impish-security universe
deb http://security.ubuntu.com/ubuntu impish-security multiverse
# deb-src http://security.ubuntu.com/ubuntu impish-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.

修改后：（我这里使用的是清华的镜像源）

#deb cdrom:[Ubuntu 21.10 _Impish Indri_ - Release amd64 (20211012)]/ impish 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/ impish main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ impish-updates main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish-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/ impish universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ impish-updates universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish-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/ impish multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ impish-updates multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish-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/ impish-backports main restricted universe multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ impish-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 impish partner
# deb-src http://archive.canonical.com/ubuntu impish partner

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu impish-security main restricted
# deb-src http://security.ubuntu.com/ubuntu impish-security main restricted
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu impish-security universe
# deb-src http://security.ubuntu.com/ubuntu impish-security universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu impish-security multiverse
# deb-src http://security.ubuntu.com/ubuntu impish-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.

对比发现：只不过是把每个help.ubuntu.com（国外）替换成了mirrors.tuna.tsinghua.edu.cn，（清华镜像网址）。

修改完软件源后，更新软件列表和软件：

sudo apt update
 
sudo apt upgrade

对于其他版本的Ubuntu也可照着修改，只需改网址就行。