First, it's really a bad idea to use Drupal 8 beta version on production site like me. Because Drupal 8 beta release do not provide a way to upgrade between beta versions. The database and API are not frozen at all.
If you are unlucky like me, you can try following steps:
- Download beta 10 and do a fresh install.
- Download beta 6 database through phpMyAdmin with INSERT IGNORE option.
- Import beta 6 database to beta 10. You will get some table stucture issues about menu and user table. Comment those lines out.
- install latest drush. run `drush cr` to rebuild cache
- open the site. You will get taxonomy_term_reference issue. Hack relative code to ignore syntax errors.
I am not sure whether there are other things need to do. I plan to upgrade my production site when stable version released. I hope my articles' data can be preserved then :(