在Ubuntu系统中，可以通过编辑 `/etc/apt/sources.list` 文件来配置软件源。打开终端，输入以下命令：
```
sudo nano /etc/apt/sources.list
```
在打开的文件中，可以看到已经配置的软件源。可以根据需要注释或者取消注释相应的源。
常用的软件源包括官方源、阿里云源、清华大学源等。
官方源配置示例：
```
deb http://archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://security.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse
```
阿里云源配置示例：
```
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
```
配置完成后，保存文件并退出编辑器。然后在终端中执行以下命令更新软件源：
```
sudo apt update
```
现在，系统的软件源已经配置完成。可以使用 `apt` 命令安装、更新、卸载软件包了。