I'm located in China and the mirrors elsewhere seem extraordinarily slow, so I'd like to switch to the one which is hosted over the road from me.
I switched it in /etc/apt/sources.list, but when I try to also change it in /etc/apt/sources.list.d/raspi.list, I get:
W: GPG error: http://mirrors.tuna.tsinghua.edu.cn/debian stretch Release: The following signatures couldn't be verified because the public key is not available
Any idea how to fix this problem?
Max.