apt-get upgrade upgrades my local MySQL instance on my Ubuntu laptop I get the following error:
(1698, "Access denied for user 'root'@'localhost'")
The fix each time is the following, so here it is for me next time save me wasting time googling the error every time.
sudo mysql -u root use mysql; update user set plugin='mysql_native_password' where User='root'; flush privileges;
And with that all is well again!