when you update you can get errors like this:
The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 6AF0E1940624A220

you can solve it by doing:
sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys (key number)
ex:
sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys 6AF0E1940624A220