修改/etc/apt/sources.list，去掉自带的源，添加如下源：

deb http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ buster main contrib non-free rpi
deb-src http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ buster main contrib non-free rpi

修改：/etc/apt/sources.list.d/raspi.list，去掉自带的源，添加如下源：

deb http://mirrors.tuna.tsinghua.edu.cn/raspberrypi/ buster main ui
deb-src http://mirrors.tuna.tsinghua.edu.cn/raspberrypi/ buster main ui

FINAL

sudo apt-get update && apt-get upgrade