Sometimes a manually added PPA could cause update error which looks like below during the update:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXXXX
This can be fixed by issuing the following 2 commands in terminal:
gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv XXXXXXXXXX
gpg --export --armor XXXXXXXXXX | sudo apt-key add -