Change mysql root password on Centos7

Stop Mysql

 systemctl stop mysqld 

Set Mysql environment option

 systemctl set-environment MYSQLD_OPTS="--skip-grant-tables" 

Start Mysql

 systemctl start mysqld 

Login to Mysql as root

 mysql -u root 

Update the root user password

 mysql> UPDATE mysql.user SET authentication_string = PASSWORD('MyPassword') -> WHERE User = 'root' AND Host = 'localhost'; mysql> FLUSH PRIVILEGES; mysql> quit 

Stop mysql

 systemctl stop mysqld 

Unset the mySQL envitroment option so it starts normally next time

 systemctl unset-environment MYSQLD_OPTS 

Start mysql

 systemctl start mysqld 

Login to mysql again using your new password:

mysql -u root -p

NB : for 5.7.6 and later, you should use
mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY 'MyNewPass';