1. Open & Edit /etc/my.cnf
  2. Add skip-grant-tables under [mysqld]
  3. Restart Mysql
  4. You should be able to login to mysql now using the below command mysql -u root -p
  5. Run mysql> flush privileges;
  6. Set new password by ALTER USER 'root'@'localhost' IDENTIFIED BY 'NewPassword';
  7. Go back to /etc/my.cnf and remove/comment skip-grant-tables
  8. Restart Mysql
  9. Now you will be able to login with the new password mysql -u root -p

source: https://stackoverflow.com/questions/41645309/mysql-error-access-denied-for-user-rootlocalhost