117笔记问答要更新Ubuntu 18.04的软件源为国内源，可以按照以下步骤进行操作：
- 打开终端，通过以下命令备份当前的软件源列表文件：
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
- 编辑软件源列表文件，使用以下命令打开编辑器：
sudo nano /etc/apt/sources.list
- 在打开的文件中，删除所有内容，然后粘贴以下内容，这是一个推荐的国内源列表：
# 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-src http://archive.ubuntu.com/ubuntu/ bionic main restricted # 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 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 # 预发布软件源，不建议启用 # 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
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保存更改并关闭编辑器。对于Nano编辑器，请按Ctrl + X，然后按Y，最后按Enter保存文件。
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运行以下命令更新软件源：
sudo apt update
现在，您的Ubuntu 18.04系统的软件源已更新为国内源。您可以使用apt命令安装和更新软件包，速度应该会更快。