要更新Ubuntu 18.04的軟件源為國內源，可以按照以下步驟進行操作：

1. 打開終端，通過以下命令備份當前的軟件源列表文件：

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

2. 編輯軟件源列表文件，使用以下命令打開編輯器：

sudo nano /etc/apt/sources.list

3. 在打開的文件中，刪除所有內容，然后粘貼以下內容，這是一個推薦的國內源列表：

# 默認注釋了源碼鏡像以提高 apt update 速度，如有需要可自行取消注釋
# deb-src http://archive.ubuntu.com/ubuntu/ bionic main restricted

# 默認注釋了源碼鏡像以提高 apt update 速度，如有需要可自行取消注釋
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted

deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted

deb http://mirrors.aliyun.com/ubuntu/ bionic universe
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic universe
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates universe
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates universe

deb http://mirrors.aliyun.com/ubuntu/ bionic multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates multiverse

deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse

# 預發布軟件源，不建議啟用
# deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted
deb http://mirrors.aliyun.com/ubuntu/ bionic-security universe
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security universe
deb http://mirrors.aliyun.com/ubuntu/ bionic-security multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security multiverse

4. 保存更改并關閉編輯器。對于Nano編輯器，請按Ctrl + X，然后按Y，最后按Enter保存文件。

5. 運行以下命令更新軟件源：

sudo apt update

現在，您的Ubuntu 18.04系統的軟件源已更新為國內源。您可以使用apt命令安裝和更新軟件包，速度應該會更快。