I have my personal memo as:

my.cnf

[mysqld]
.
.
skip-grant-tables
.
------------------------------------
restart

mysql -u root
mysql> use mysql
Database changed

mysql> UPDATE user SET authentication_string=password('    ') WHERE user='root';
Query OK, 1 row affected, 1 warning (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 1

mysql> flush privileges;

and the similar thing may be found at the diary 2019/02/16 .

But this entry can not be found with the search by 'mysql'. I needed to use 'mysqld' instead.