1. stop the mysqld daemon
    sudo systemctl stop mysql.service # it depends on your distro
  2. start mysql server skipping grant tables
    mysqld_safe --skip-grant-tables &
  3. start mysql client
    sudo mysql
  4. run the following sql commands
    UPDATE mysql.user SET Password=PASSWORD('NEW-PASSWORD') WHERE User='root';
    FLUSH PRIVILEGES;

Tutorial from official docs