You can easily change a WordPress user password programmatically by utilizing WordPress wp_update_user function.

See the following PHP codes to learn how to use it.

$user_id = 'a WordPress user ID';
$pass_new = 'new password for the user';
wp_update_user(
    array (
        'ID' => $user_id, 
        'user_pass' => $pass_new
    )
);