更换源后报错：
E: Failed to fetch http://mirrors.tuna.tsinghua.edu.cn/ubuntu/dists/trusty-security/main/binary-arm64/Packages 404 Not Found [IP: 2402:f000:1:400::2 80]
E: Failed to fetch http://mirrors.tuna.tsinghua.edu.cn/ubuntu/dists/trusty-updates/main/binary-arm64/Packages 404 Not Found [IP: 2402:f000:1:400::2 80]
E: Failed to fetch http://mirrors.tuna.tsinghua.edu.cn/ubuntu/dists/trusty-backports/main/binary-arm64/Packages 404 Not Found [IP: 2402:f000:1:400::2 80]
E: Failed to fetch http://mirrors.tuna.tsinghua.edu.cn/ubuntu/dists/trusty-proposed/main/binary-arm64/Packages 404 Not Found [IP: 2402:f000:1:400::2 80]
E: Failed to fetch http://mirrors.tuna.tsinghua.edu.cn/ubuntu/dists/trusty/main/binary-arm64/Packages 404 Not Found [IP: 2402:f000:1:400::2 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.
原因：cpu是arm架构，基本都是x86的源的配置，用错源自然找不到所需的包裹信息，报404错误。
更改为：
# 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 https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-security main restricted universe multiverse
# 预发布软件源，不建议启用
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-proposed main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-proposed main restricted universe multiverse
