在Ubuntu系統中，可以通過編輯 `/etc/apt/sources.list` 文件來配置軟件源。打開終端，輸入以下命令：

```
sudo nano /etc/apt/sources.list
```

在打開的文件中，可以看到已經配置的軟件源。可以根據需要注釋或者取消注釋相應的源。

常用的軟件源包括官方源、阿里云源、清華大學源等。

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，保存文件并退出編輯器。然后在終端中執行以下命令更新軟件源：

```
sudo apt update
```

現在，系統的軟件源已經配置完成。可以使用 `apt` 命令安裝、更新、卸載軟件包了。