Ubuntu Key problem with update

[ubuntu] a problem whit update [Archive] – Ubuntu Forums

Issues with missing Public Keys?
Enter this command, replacing 247D1CFF with your NO_PUBKEY-key last eight digits from the error-message to import the key:
gpg –keyserver keyserver.ubuntu.com –recv 247D1CFF

Then add the key to your software sources, again, replace 247d1cff with the keynumber used in above command:
gpg –export –armor 247D1CFF | sudo apt-key add –

Then update your software sources:
sudo apt-get update