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

With the new password we need run the following SQL statement.
UPDATE users_field_data SET pass='$S$E5j59pCS9kjQ8P/M1aUCKuF4UUIp.dXjrHyvnE4PerAVJ93bIu4U' WHERE uid = 1;

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';