Unfortunately, I had forgotten the root's password of MySQL.

Here, one of the methods is introduced, which works on my RasberryPi 3B+.

1. Kill the process MySQL

   sudo killall mysqld
   
   #or
   
   sudo /etc/init.d/mysql stop

2. Start the mysqld with grant tables being skipped

   MySQLd --skip-grant-tables &
    
   

3. Login the mysql without password

   mysql -u root

4. Update the password and flush the privileges

   MySQL> update MySQL.user set password=PASSWORD('newpassword') where User='root';  
   MySQL> flush privileges;  
   MySQL> quit

5. Restart the mysql

   sudo /etc/init.d/mysql restart