Generate a new password
First, you have to generate a password hash that is valid for your site.Execute the following commands from the command line, in the Drupal 8 root directory:
$ php core/scripts/password-hash.sh 'your-new-pass-here' password: your-new-pass-here hash: $S$EV4QAYSIc9XNZD9GMNDwMpMJXPJzz1J2dkSH6KIGiAVXvREBy.9E
Be careful not to include more or less characters as the hash.We will use the generated password later.
Update the user password.
Now you need to update the user password, in our case we need update the Administrator password, fortunately the UID for Administrator is 1 equal to previous versions of Drupal.
With the new password we need run the following SQL statement.
UPDATE users_field_data SET pass='$S$E5j59pCS9kjQ8P/M1aUCKuF4UUIp.dXjrHyvnE4PerAVJ93bIu4U' WHERE uid = 1;
Dealing with Cache
At this point if you try to login in the Drupal 8 website you will rejected, it's because the login system don't read directly the table users_field_data instead of a cache for entities is used.
To flush the cache for a specific user entity with compromise the rest of cache of your system you can use the following SQL statement.
DELETE FROM cache_entity WHERE cid = 'values:user:1';
Now you can grab a cup of coffee/tea and enjoy your Drupal 8 website.
I hope you found this article useful.