/etc/init.d/mysql stop

[ ok ] Stopping mysql (via systemctl): mysql.service.
[[email protected]] ~ #mysqld_safe --skip-grant-tables --skip-networking &
[1] 25335
[[email protected]] ~ #160619 00:45:55 mysqld_safe Can't log to error log and syslog at the same time. Remove all --log-error configuration options for --syslog to take effect.
160619 00:45:55 mysqld_safe Logging to '/var/log/mysql/error.log'.
160619 00:45:55 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql

mysql mysql -uroot
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.5.49-0+deb8u1 (Debian)

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affiliates. Other names may be trademarks of their respective

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql>UPDATE user SET password=PASSWORD("MYNEWPASSWORD") WHERE User="root";
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2  Changed: 2  Warnings: 0
Query OK, 0 rows affected (0.00 sec)
mysql> \q