First step is to stop MySQL service.

sudo /usr/local/mysql/support-files/mysql.server stop
Then you need to start it in safe mode

sudo /usr/local/mysql/bin/mysqld_safe --skip-grant-tables
secondly: let's open another shell/terminal window, log in with no password

  mysql -u root
UPDATE mysql.user SET Password=PASSWORD('my-new-password') WHERE User='root';
FLUSH PRIVILEGES;
\q
Because in MySQL 5.7, the password field in mysql.user table is removed, now the field name is 'authentication_string'.

 mysql -u root
UPDATE mysql.user SET authentication_string=PASSWORD('my-new-password') WHERE User='root';
FLUSH PRIVILEGES;
\q
Now again yu need to start the MySQL server

sudo /usr/local/mysql/support-files/mysql.server start