Comment 5 for bug 1916672

Xenial running mysql version 5.6.37-82.2. The syntax is as follows for mysql database server version 5.7.5 or older:

SET PASSWORD FOR 'nagios'@'localhost' = PASSWORD('new-password');

OR

UPDATE mysql.user SET Password=PASSWORD('new-password') WHERE USER='nagios' AND Host='localhost';

https://www.cyberciti.biz/faq/mysql-change-user-password/