系统换源后更新，出现以下问题：
Get:1 http://mirror.tuna.tsinghua.edu.cn/raspberrypi buster InRelease [32.6 kB] Get:2 http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian buster InRelease [15.0 kB] Get:3 http://mirror.tuna.tsinghua.edu.cn/raspberrypi buster/main Sources [111 kB] Err:2 http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian buster InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9165938D90FDDD2E Get:4 http://mirror.tuna.tsinghua.edu.cn/raspberrypi buster/main armhf Packages [393 kB] Get:5 http://mirror.tuna.tsinghua.edu.cn/raspberrypi buster/main arm64 Packages [304 kB] Reading package lists... Done W: GPG error: http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9165938D90FDDD2E E: The repository 'http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian buster InRelease' is not signed. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9165938D90FDDD2E
使用以下命令进行签名：
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 9165938D90FDDD2E
再次 apt-get update 即可
