sudo apt install leafpad( "_* I prefer to use it)
sudo leafpad /etc/apt/sources.list
deb https://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ buster main contrib non-free rpi
& another one:
leafpad /etc/apt/sources.list.d/raspi.list
deb https://mirrors.tuna.tsinghua.edu.cn/raspberrypi/ buster main
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If you only change one,sometimes you will get update erro.
