MySQL - How to change root user's password

(1) Create a custom initialization file containing this (e.g. named custom-init.txt):

ALTER USER 'root'@'localhost' IDENTIFIED BY 'YOUR_NEW_PASSWORD';

(2) Start MySQL using this command: 

mysqld --init-file=C:\\mysql\custom-init.txt

PS. Variants of the alter user statement may be necessary for MySQL versions earlier than 5.7