Changing the password of Drupal 8 user is easily done using drush. As of this post, the Drush version I’m using is version 9.5.2 and was installed in the project’s directory using composer (composer require drush/drush).
The syntax is:
drush upwd [user] [password]
drush user:password [user] [password]
For example, to change the admin user password:
drush upwd admin "another_password"