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 InReleaseThe 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