I try to update system Linux 17 and I get the answer "system is ok" but there is a note saying that there is no key for 1397BC53640DB551. I cannot open a number of flash files though I have tried via Adobe. What to do?
Answer to the question
A web search yielded the following taken from http://askubuntu.com/questions/766883/there-is-no-public-key-available-for-the-following-key-ids-1397bc53640db551.
In a terminal, copy and paste the following:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1397BC53640DB551