盼星星盼月亮，LMDE4终于来了。官方也给出了升级教程：
https://community.linuxmint.com/tutorial/view/2475
然而在执行mintupgrade download时，下载速度慢出翔！在执行命令时打开
/etc/apt/sources.list.d/official-package-repositories.list
瞧了一眼，mintupgrade强行把源改成了官方源。
那就照猫画虎手动换成国内源呗，比如清华
deb https://mirrors.tuna.tsinghua.edu.cn/linuxmint debbie main upstream import backport #id:linuxmint_main
deb https://mirrors.tuna.tsinghua.edu.cn/debian buster main contrib non-free
deb https://mirrors.tuna.tsinghua.edu.cn/debian buster-updates main contrib non-free
deb https://mirrors.tuna.tsinghua.edu.cn/debian-security/ buster/updates main contrib non-freedeb https://mirrors.tuna.tsinghua.edu.cn/debian buster-backports main contrib non-free
随便找个文本编辑器（需要root权限）编辑
/usr/share/linuxmint/mintupgrade/apt_destination_sources
把这堆东西替换掉官方源就妥了。
