For example:
W: GPG error: http://ppa.launchpad.net trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 93C4A3FD7BB9C367
go to http://keyserver.ubuntu.com/
search your key 0x93C4A3FD7BB9C367
Search results for '0x93c4a3fd7bb9c367'
Type bits/keyID Date User ID
pub 4096R/7BB9C367 2014-06-10 Launchpad PPA for Ansible, Inc. Fingerprint=6125 E2A8 C77F 2818 FB7B D15B 93C4 A3FD 7BB9 C367
go to http://keyserver.ubuntu.com/pks/lookup?op=get&search=0x93C4A3FD7BB9C367
save to file public key and add it to system with command
sudo apt-key add <file-with-key>