下列签名无效: EXPKEYSIG 529F6EF08FA0B9DF

图形界面-软件和更新,选择更新源中国
其它软件-取消报错的源
再执行apt-get update 

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 8C718D3B5072E1F5

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 529F6EF08FA0B9DF
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 529F6EF08FA0B9DF

密钥存储在过时的 trusted.gpg 密钥环中(/etc/apt/trusted.gpg)

cd /etc/apt/
sudo cp trusted.gpg trusted.gpg.d

sudo rm -f /etc/apt/trusted.gpg.d/trusted.gpg
sudo cp /etc/apt/trusted.gpg /etc/apt/trusted.gpg.d